I've used this lens alongside other Canon consumer grade and "L" lenses. Of course, the Ls put this lens to shame, as you'd expect. But against other consumer grade lenses, it holds up pretty well. As others noted, it's weakest at 200-300mm, but even there, my copy of the Sigma equals the Canon 75-300 lenses in sharpness and chromatic aberration. You do have to be careful with technique (watch the ISO and shutter speeds), and you should also make sure your copy of the lens is focusing properly. I've had to return several Sigma lenses with front or back-focusing, but eventually you get a good copy.
The Sigma is hard to beat for compactness and convenience, and even the Tamron 28-300 XR is noticeably fuzzier at 300mm. The Tamron also lacks the Sigma's metal mount, and costs more.
This lens is pretty sharp below 200mm. For serious shooting at 300mm, you should pay for something bigger and heavier. Pictures at 300mm, when examined pixel-by-pixel, will lack sharpness. You won't be able to crop and enlarge such photos, and you'll need a lot of sunlight because f6.3 is rather slow. You'll have a hard time photographing flying birds at 300mm because the autofocus won't keep up. If you do any of those things, you should get something else.
But within limitations, and with some post-processing, photos at 300mm can look quite good at sizes up to 8x10", basically equal to the Canon 75-300 consumer zoom family. In fact, I would say the Sigma is optically better, with less purple fringing around extremely bright objects. If you're at all considering the consumer-grade 75-300 zooms, I'd suggest looking at this lens also.
The bokeh at wide open apertures can be donut-shaped. I don't see this often, but it's annoying when it does appear.
Another review compared this lens to having a $400 big-zoom digicam. I have one of those digicams, and this lens is hugely better in optics, shutter lag, noise. In my hands, the comparison isn't even close.
My biggest complaint is with autofocus, which occasionally hunts even in the best of lighting conditions. A second complaint is that 28mm isn't very wide considering the DSLR crop factor. Nonetheless, within limitations, this lens is quite handy.