I had a chance to quickly test mine. Sharpness is nice, slightly beats my Tamron in the lower FL ranges, and closely matches Tamron, maybe a tiny bit less sharp at 600mm, which accurately mirrors the lens tests and reviews I've seen online. I think sharpness at 600mm in the real world (with its haze, heat, etc.) would be roughly the same.
Build/finish is really awesome; my Tamron is pretty good too but I think the Sigma is a class above. AF speed is about the same, but has more/better limiter options.
OS is not as good as I thought, my experience is similar to Jake's, I think my Tamron does a little bit better job.
I'm not sure yet if I'm going to keep it. I've became emotionally attached to my Tamron, has been with me in Europe for 3 weeks last summer and did a great job. I also like the manual focus ring and operation a bit better on my Tamron (it's wider), even though I hardly ever use it.
Although I still own my Sigma 1.4x TC from my 120-300 2.8 OS days, I have only used it once on my Tamron for the past year (for Moon shots), so to me the fact that the Sigma works with TCs and the Tamron won't, has no real value to me.
It's too early to tell what its fate is going to be, but one thing is certain: this Sigma lens certainly makes me want to buy a bunch of new generation Sigma lenses. I never had any problems with Sigma's older lenses, owned a bunch in the past and still have and use the 70-200 2.8 OS as my main wedding tele workhorse, but this new generation is really a work of "art" (even though it's a "contemporary" )