If your not as cheap as I am, I would warn you away from the cheaper model Sigmas. I've had a couple that I picked up at VERY reasonable prices, but turned out to be quite dull, even dogs. At first I thought it was the AF function, but I soon learned it was the glass. Even when I focused manually, the image just wasn't very sharp.

I shoot a 450si with a Sigma 28-105 IF that I picked up for a song...Well the damn thing vignettes at 28mm and is not very sharp at all. I've been quite disappointed and shopping for a replacement...