We quickly upgraded to this lens after finding the 70-300mm kit lens was just too short. After agonising for a while over various Canon offerings (300mm, 400mm, 100-400mm) all in the £1000+ range, and the Tamron 200-500mm not far behind, the lure of image stabilised 500mm for far less than a grand was too much and we opted for the Sigma.
Compared to the 70-300mm this is definitely a much bigger, heavier and longer lens, as you might expect. With the included hood attached and a neoprene camouflage cover it looks a lot more impressive than it actually is but it has served well as our birding lens for around 2 years. While heavy it is hand-holdable for minutes at a time and can be carried around large reserves or medium hikes without being too much of a burden. We found a light aluminium monopod and monopod head (eg Manfrotto 234RC) can just support the weight, including a pro-sized body, although something more chunky will give more stability.
First, the good points. This lens is about the only way you can get 500mm of focal length (or 800mm on an APS-C body like the 50D) with image stabilisation, for less than a thousand, with full autofocusing. With prime 500mm lenses costing at least £3k or more, and various shorter fast lens plus teleconverter combos being not much cheaper, getting decent reach for small birds is a case of either large compromises or even larger prices.
The sharpness, colours and contrast are certainly a lot better than the 70-300mm. From shoots where both were in use at the same time, the 70-300mm shots nearly always got trumped by the Sigma, even where the focal lengths being used were comparable. The image stabilisation is very effective (I haven't measured it but it's specified at 4 stops, the same as the newer Canon IS implementations) for such a long lens and it gets needed a lot with most of our shots being hand held.
For such a long lens it has quite a close minimum focusing distance, giving a magnification of 0.17x. This has made it reasonably useful for large insects eg dragonflies and butterflies, who are too skittish for a 100mm macro lens.
As is usual with Sigma, not only are the lenses a lot cheaper than Canon equivalents, but you get a lot of extras included. In this case a lens hood, carry case, tripod foot and strap are included. Apart from the case these have all been used nearly constantly.
Now, inevitably for a cheap telephoto, the bad points. The maximum aperture of f/6.3 at 500mm is just too slow. Coupled with a camera that isn't stellar at high ISOs this leads to almost constant lack of available light to keep shutter speeds up and noise down. This increases frustration in dim weather, something we seem to get a lot of in this country, where you're struggling with noise obscuring detail and slow shutter speeds blurring motion (IS can't help there).
To make matters worse the lens is quite soft at maximum aperture. We see a definite improvement going from f/6.3 to f/7.1 and for best quality we use f/8.0. Although good results can be obtained from this lens it does need a lot of light to do so. Compare this to a 500mm prime at f/4-4.5 and we're 2 good stops slower, stops you can rarely afford to give up.
While the autofocus is quiet and reasonably quick for such a big lens, it certainly doesn't compare in speed or accuracy to more expensive prime lenses. Compared to other lenses on the same body, at similar focal lengths, there are a lot more misses. The small aperture with deep depth of field masks much of it but where sharpness is crucial it's not as reliable as I would expect.
A minor nitpick is the level of noise from the image stabilisation. Although not too loud in operation it does have a very obvious CRUNCH noise when starting or stopping. It's unlikely to scare a bird more than the shutter noise on an SLR but the shutter only goes when you're taking the shot.
The other nitpicks are common to most zooms. The zoom mechanism can pull in dust over time and we've got a little in the barrel now. The barrel can extend/retract when held vertically, although there is a lock to keep it at 150mm. Although marketed as 150-500mm most people find it's significantly shorter at the 500mm end, probably more like 480mm. This isn't a major difference but when you can never have enough focal length it is niggling.
Still, over the years we've taken a lot of pleasing shots with the lens and it didn't cripple our bank account (or us carrying it). We never expected it to be the best birding lens ever and it's performed well given those expectations. The writing was on the wall for a while though when comparing to shots on other lenses, and a rented Canon 500mm was the final seal of doom. One way or another we felt we needed to upgrade to something faster and sharper but without losing the focal length.
Although its place as primary birding lens has since been replaced by a Canon 500mm prime lens we're unlikely to get rid of it just yet. While the Canon 500mm is worlds apart for image quality, autofocus speed, accuracy and light gathering it is a big, heavy monster with a very long close focusing distance. One of the shorter, better quality Canon or Sigma lenses will probably replace it eventually, but there's no rush.