hey Helen, hope i am not too late to the party, i personally own the sigma 150-600 sports and for me it's been nothing short of phenomenal (for the price)

being a 3rd party 150-600 super zoom my expectations weren't actually that high, i just wanted a reasonably priced 600mm lens (got mine brand new on sale for $1700) but when i actually got to use it the lens blew my expectations out of the water

sharpness wise it is fantastic wide open at 600mm, is it the sharpest lens i've used? no definitely not, but it's not far off

auto focus wise it is very accurate, it doesn't focus as fast as my 70-200L 2.8 II or 24-70L 2.8 II (and it would be unfair to compare) but it's not far off, it's actually quite fast (and the recent firmware update has made it faster) but it's supremely accurate, it just find ways to nail focus, easily comparable to the 2 lenses i've mentioned (maybe i just got lucky and received a very good copy) accuracy wise

now the lens is heavy, and depends on your fitness it may be an issue, for me personally i am a pretty average built guy and i have not problem carrying it around with me for hours at a time simply using a r-strap, i shoot it 100% hand held, the OS works well i've gotten sharp shots at 1/50 shutter speed at 600mm out in the field, and not uncommon for me shooting at 1/100-150 speed when light gets low, you could always consider the C version which is a lot cheaper and lighter and from what i've seen not much difference image quality wise

House Sparrow by Tony, on Flickr

Superb fairy wren juvenile by Tony, on Flickr

Spotted Pardalote by Tony, on Flickr

Scarlett Honeyeater by Tony, on Flickr