Just had a question about the Sigma 120-300mm and whether I'd recommend it. Here's my answer:

There's absolutely loads on the web about this lens and most of it is good. It's sharp, AF speed is reasonable and colour rendition/contrast is fine. It's great to have a range of focal lengths available with the one lens. It works well with teleconverters both 1.4 and 2x and supposedly works best with Sigma TCs. I've used both Canon and Sigma TCs.

I'm not entirely happy with mine although I've hardly used it. I'm not entirely sure the 4 stop IS is working consistently and yesterday the AF didn't work at all until I focused manually a couple of times and then it behaved itself. This ties in with the negative comments about the lens - ie reliability issues. This is countered to a certain extent by a 3 yr warranty from Sigma.

If money was no object I'd have the Canon 300mm f.28 or even better the 200-400mm when it comes out. If time had been no object, I'd have waited for the new Sigma 120-300 with a focus limiter switch and improved production methods so hopefully better reliability.