Just got the Tamron 15-30 f2.8 Di VC yesterday. This lens has some serious field curvature which helps it in many landscape and interior situations. This is possibly why so many like it as it can get the grass at your feet in focus as well as the subject matter in the mid to med distance in focus. This is not an uncommon trait of super-wide angle lenses and not necessarily a bad thing. All super-wide angle lenses need to compromise as they all exhibit some form of field curvature in some degree, just how much depends on what the lens designer wanted the end result to be. The wider you go, the more difficult it is to make a lens that is all things to all people, you just have to learn the lens's foibles and strenghths and play to them.
It is early days yet as Ive only shot with it for a few hours yesterday so, is it better than the Nikon 16-35 f4 VR? Tough question as it behaves differently in that with all that filed curvature you can get more in focus with a normal landscape or interior photo. It's like taking a photo of the inside of a bowl and getting the leading edge lip and all the way to the bottom of the bowl all in focus, however, take a photo of a wall and you will see the edges drop off in sharpness. Anyway, as I said, I've only had it for a few hours and what is sharp is very sharp.
Colour balance is very cool, cooler than the Nikon 16-36, 14-24 and 24 f1.4 that I use for this end of the focal range. I like the Nikon colours better, but some tweaking in post process fixes things up, mostly.
VR seems to work very well. I tried it down to 1/10sec and the photos were tack sharp.
It feels and handles similarly to the 14-24 in that it is a little front heavy. Not a big deal but it is still quite heavy, much heavier than the 16-35 which is a consideration for when you want to travel with it. However, I looked at this way, I would normally have taken the 16-35 and the 24 f1.4. With the Tammy, I won't require the 24 f1.4 saving room and a small amount of total weight.
D810 + 15-30 f2.8 Di VC @ 18mm, f10, 1/250sec, ISO100.
D810 + 15-30 f2.8 Di VC @ 15mm, f10, 1/250sec, ISO100.
D810 + 15-30 f2.8 Di VC @ 15mm, f8, 1/10sec, ISO1000.
D810 + 15-30 f2.8 Di VC @ 15mm, f2.8, 1/10sec, ISO64.
D810 + 15-30 f2.8 Di VC @ 15mm, f8, 1/10sec, ISO1600.
D810 + 15-30 f2.8 Di VC @ 15mm, f8, 1/15sec, ISO720.