I’m very happy with my Sigma, and wouldn’t hesitate to recommend it; I wouldn’t be without a telephoto now that I have one. However, based on my limited experience I think ImageSanguine makes a good point about the IS factor. I suspect the softness of my telephoto shots is because the lens I own doesn’t have Image Stabilization.
For me it was a cost factor; I simply couldn’t afford to invest in a higher end lens, even though I know that’s the key to better photos. If cost isn’t an issue I imagine going IS in a lens is always a good idea when the camera itself doesn’t have it. (My rebel doesn’t.) Over time I’ll be able to build up a better lens collection, and probably move to a camera with IS. For now, I’m lucky to have what I have.
I’m vey honoured that ImageSanguine has been in contact with me. Check out their Flicker photostream, and you’ll see why! What an eye for colour!