I thought I'd share some thoughts about using big telephoto lenses. This is written from the perspective of a Canon owner who mainly does bird photography, but it should be readily generalisable to Nikon gear, and have some relevance to other types of photography such as sport.
I've owned the Canon 500mm f/4L for nearly three years now, and it's the only big lens I've had the opportunity to do more than play briefly with, but I think most of my comments should apply fairly well to others in the same general class. I'll define "big lens" as anything beyond the usual 400mm f/5.6 or 500mm f/6.7 class. Canon make four big white prime lenses with IS: 400/2.8, 500/4, 600/4, and 800/5.6. Nikon now have a matching range of big primes with VR, plus the much smaller 200-400/4 VR, which is nevertheless bulky enough and heavy enough to place in the same category. I'll not consider the much smaller 300mm f/2.8 lenses or the Canon 400/4 DO - I suspect that they are very different beasts, and deserve their own mini-article. (If you'd like to read it, just send me, oh, say a 400/4 DO and a 300/2.8 to try out. I'll send them right back to you as soon as I've finished testing. That will be around this time in 2011, give or take.)
The first thing most photographers notice about a big white is the price. Forget the price! If you want one, you can get one. You'll have to save up for a while and go without some of the useless consumer junk you usually buy (new TV sets and curtains and all that other stuff you normally waste hard-earned dollars on), and start making sandwiches for lunch instead of getting take-aways, but so what? When was the last time your new Jag jeans or your Playstation took a decent photograph? Just decide that you are only going to spend money on stuff that matters. Lenses matter. Almost anyone can afford a 600/4 - you just have to decide where your priorities are. If you are serious, you can find a way. If you can't find a way, you are not serious. (And that's OK too. There is no law saying you have to be a fanatic if you don't want to be.) So, forget the price.
The second thing photographers ask, and the first thing non-photographers ask, is "how close do I have to be to the bird with that monster?" Answer: closer than you think. The difference between, say, a 100-400 and a 500/4 is not all that much so far as zooming in on the bird goes, and is easily swallowed up by other factors, such as different camera bodies. For example, my usual favourite setup of 1D III and 500/4 has less reach than a 50D and a 100-400, or about the same reach as a 100-400 with a 20D or 40D. So the second thing to forget is the amazing reach of a true 500mm lens .... because it offers no such thing. You still have to get close to the bird - and because the lens is so much bigger and heavier and more visible, that's harder than it was with a 400/5.6. (More on this a little later.)
But there is one reach advantage: the very first thing you do when you finally get a big f/4 lens is buy a teleconverter. With a 1.4 converter, a 500/4 becomes a 700/5.6; and a 600/4 becomes an 840/5.6. Now we are looking at some serious reach! Until you upgraded from (say) a 100-400/5.6 you couldn't get close to those focal lengths without going to manual focus and horribly slow shutter speeds. But the moment you put a converter on, paradoxically, you take away a lot of the reason for using a big white in the first place. I better explain ......
The biggest advantage of a big white over a smaller super-tele isn't reach: it's speed. If you do wildlife (or probably sport for that matter), fast, accurate focus generally matters a lot more than ultimate reach. If you care enough about doing wildlife photography to have gone without all those take-aways and bought a big white, then you probably cared enough about it to learn some fieldcraft with your smaller lens, and you are probably pretty good at getting close enough to the bird to get a good result. OK, you never have as much reach as you want, but you can get by with 400mm. (Just!) You can't get by without accurate focus, and even when it's accurate, slow focus is often useless. With wildlife, you either seize the moment or you miss it. Bare lens, the 500/4 has incredibly fast and accurate focus. I have no reason to doubt that the other big whites are just as good, and that the same goes for the big new Nikons.
But the speed advantage I'm talking about isn't just focus speed. Even more important is the ability to shoot at f/4. The ability to select a narrow a depth of field quite often makes composition much more rewarding - that creamy blurred-out background to a bird photograph didn't get to be the standard ideal by accident, like most cliches, it became a cliche because it works so well.
And the ability to work in low light is priceless. If you want to work in rainforest or tall eucalypt forest, f/4 isn't handy, it's essential, and f/2.8 would be even better. (Many times doing rainforest work I have wished for an extra grandmother to trade-in on a 400/2.8.) If your birding lens maxes out at f/5.6, deep forests are pretty much a no-go zone unless you resort to blasting everything with flash. At f/4, it can still be pretty tough, but it's do-able. Obviously, you are having to work with very shallow DOF and your focus needs to be spot on, but these are things you can work with and overcome: too little light to get decent colour and shutter speed is something you can't overcome.
As soon as you put a teleconverter on your big white lens, you throw away those twin advantages. Yes, you get extra reach, but your focus speed drops to something not much faster than a 100-400 (and probably slower than a 400/5.6, though I've never tried one of those out for myself), and you are at f/5.6 or higher. You also lose a little bit of contrast, a little bit of colour, and your out-of-focus background isn't as attractive as before. On the big primes at least, your base image quality is so good that you can afford to waste a little of it, but it's something worth considering just the same. When you go to a 2X converter, these changes start to become quite significant. I hardly ever use my 2X, though I guess I use the 1.4X maybe a bit less than half the time.
One other thing to mention before I leave this subject is, for want of a better word, tolerance. The big white primes have an extraordinary ability to tolerate sub-optimal conditions - at least the 500/4 IS does, no reason to think an 800/5.6 IS or a 600/4 VR would be any different. Mostly here I'm thinking about marginal light: not just low light, but poor quality light - overcast days or shade for example. No lens can turn an overcast into crisp winter morning sunlight, but the very best lenses seem to have a magical ability to turn in decent results even when the light isn't helping. I suspect that this applies to any top-quality lens, from wide-angle on up, but I mostly notice it with the 500. Perhaps if I owned something like an 85/1.2 or a 35/1.4 I'd see it there too.
Oh, and you need to be aware of the minimum focus distances: the big primes all have horribly long MFDs - 4.5 metres for the 500/4, worse than that for most of the others. This means that for small birds you have to muck about with close-up rings which are a complete pain as you forgo infinity focus and the moment you stop and put one on, some larger creature will pop up and taunt you from too far away to get a focus on! For this reason, I often carry an extra body and the 100-400 as well as the 500/4. And that leads me to my next topic.
I was trying to consider the things you notice in the order you notice them before I sidetracked myself. When you pick up a big white and try it out, the impression of sheer size is alarming, and the weight is a fair-dinkum shocker. I still remember my first day with the 500/4 (only a moderate size lens by big iron standards): I genuinely wondered if I'd be able to cope with the weight of it. It was so damn hard to hold it steady, and even quite a short walk was enough to demonstrate that trotting here and there in the carefree manner you do with a 100-400 wasn't on the agenda. A kilometre or two was sufficient to induce actual pain in the shoulders.
But you do get used to it. It takes a while, but the body acclimatises, muscles harden up, and you get smarter about ways of carrying it. Then, one day, you pick up your old 100-400 and it feels like a feather! These days, I generally draw the line at 10 kilometres in a day. Anything over about 7 or 8k and I'm a bit sore and tired. Any more than 10k and the last few kilometres are painful, and my neck and shoulder muscles give me a bit of gyp for the next day or two. So, if the walk in is long, I think pretty carefully about whether I want to do it or not before I start. Sometimes I'll take the 100-400 instead. (By the way, I'm slim and 50ish: if you are younger or have a stocky build or are just generally fitter than I am (which shouldn't be too difficult), you might be able to add a bit to my guideline distances.)
Can you hand-hold the big iron? Absolutely! I hand-hold the 500/4 about a quarter or a third of the time, use a tripod for the rest. People hand-hold 600/4s and 400/2.8s (which are half as heavy again as a 500/4 or around four times the weight of a 100-400), though they tend to be big, strong blokes, and I doubt they do it for very long at a time.
For birds, you mostly get almost as sharp a picture hand-held as you do with a tripod provided that you keep the shutter speed up, which is something I like to do anyway as I'm a big fan of shooting birds in motion, not just posing on a stick. So, in good light, the big advantage to a tripod isn't really extra sharpness, it's time. You can only hold a big white up to your eye for a certain length of time: if you think you are going to need to wait longer than that for the shot you want, then you either need a bold subject that won't mind you waving the lens around, or else to set the tripod up. With the lens on a tripod, you are pretty much tied to one spot, but you can comfortably stand and wait a lot longer for your opportunity.
Mostly, I walk slowly and quietly from likely spot to likely spot with the tripod over my shoulder, then pause and wait for as long as seems sensible (typically 5 to 20 minutes) at each spot.
The extra reach is handy, of course, but the weight and bulk of a big lens (and usually a tripod too) pretty much cancels that out. It is simply not possible to move as inconspicuously with a big, heavy lens as you do with a 400/5.6. You are slow, clumsy, your tread is heavier, you can't duck under low branches easily, and perhaps most important of all, it's very difficult to move smoothly and gracefully: you tend to jerk the thing around because it's too heavy to lift slowly. (You will see what I mean by this immediately when you try a big lens, but you can get the general idea by doing 10 quick push-ups. Not too hard? Now do them really slowly. There is your difference.)
So, in general, you wind up further away from the bird than you would have done with a smaller lens. Or else you work a fair bit harder to get as close as you want to be. Even at hides, you very often find that the viewports are too small and you can't get the front of the big lens through them.
Given all that, is it even worth the trouble? Absolutely! There are times when the extra length really comes into its own (waders are the classic example) and even with woodland birds, you can often get a good shot where you would have struggled with a 400. And the benefits of fast focus and large aperture are very rarely wasted.