I had used the non-OS version for a year, found it to be quite good after some PP. I decided to buy the OS version, sold my old one on Ebay (for $75 less than I payed for it).
The OS version is better than I expected. IQ surpasses the old version, but the OS really makes a huge difference. First, I can shoot at 200mm (320mm) at 1/30 sec, while I wouldn't dare at less than 1/300 without OS. This makes the long shots far more usable, and they come out fairly sharp. The great thing is that I can usually shoot at f8 or f11 most of the time, giving far better quality. At wider focal lengths I can shoot at 1/15 or even 1/10th!
Another great benefit of this lens is that you can shoot with natural light indoors in almost any situation. I often shoot fellow musicians performing in dark clubs, also dance concerts here in NYC. No problem for the OS Sigma.
While not a fast lens (OS makes up for it), or a lens for pixel-peepers or the elite "L" set, this is a lens for the rest of us -- and there is nothing else quite like it for Canon cameras.