This is going to sound a little wierd, but I returned my Sigma 70-200 OS today because it was a fantastic lens.
Seriously, it blew me away. Super-fast and insanely accurate AF at all focal lengths and subject distances. Sharp as a tack in every shot. The OS worked perfectly, allowing me to hand-hold 200mm at 1/30, probably lower if I had tried. The only other lens that has impressed me this much is my Zeiss Makro-Planar 100 f/2.8 ZE (for very different reasons of course).
So why did I return it?
Well, it was my first 70-200. I have wanted a 70-200 for a while but it seemed as if the only good ones were Canon's, which I was reluctant to get because I hate the attention they draw with those big white bodies. So when the newest Sigma came out that was actually supposed to AF just as well as the Canon's, and have optical quality that was at least pretty darn close, I finally figured it was time to get one....
And thus my downfall. Once I started using it I realized I love everything about 70-200's and that I would be using this thing a LOT. So much so that I realized I wouldn't be happy with the "second best" version of a lens I would be using so much.
So I returned the Sigma today and pulled the trigger on the Canon 70-200 f/2.8 IS II.
Now to just figure out how to get candid's while using a lens that screams "look at me" as loudly as it can at all times....