I have been using the non-stabilized Sigma 50-500 for a couple of years now and over all I would say I am satisfied with it. The range is fantastic, the quality is great. It has worked flawlessly for two and 1/2 years, first on the Canon Rebel XTi and now on the 60D.
It is soft at 500mm and even worse than soft if the light is not optimal. As with all big lenses, the better the light the better the image.
At roughly half the cost of the Canon 100-400 it is a bargain. If you are looking at the stabilized Sigma vs the Canon ..the price is close enough that I would buy the Canon...
I kept saving money for the Canon...and every time I saved up close to the Canon cost....the 400mm would jump in price again.. at $1,600 I just couldn't seem to justify it to myself or my management....ie wife :
Here are some fast jets taken recently on the 60D.
This is a 500mm example...
And one at 300MM
Static Jet
64MM on the Rebel