I bought the lens 3 months back based on a number of reviews, including this one here:http://webservice.gentec-intl.com/GentecInc/Awards/OS120300HS-sigma_120-300mm_review_john_e_marriott.pdf
My experience with the lens has been similar to John's - it's tack sharp right through the range, only slightly (virtually unnoticeable) less wide open. I use it on a 7D - which effectively removes the 'soft' edges. I know it takes the Canon TC's, however, I use the Sigma 1.4 and 2.0 - reviews suggest the difference between the Canon and Sigma TC's are marginal and for the cost difference I went with the TC's made for the lens. Of course you have the slower autofocus that comes with any TC and if you're far out of focus (complete blur) I have found it needs to be brought in manually (only happens in the extreme for the 1.4).
I don't have a 70-200 so can't compare - some say the Sigma is sharper, others say it's close, all I know is that it's damn sharp. AF is fast (though apparently just slightly slower than the 70-200, but again I haven't compared them first hand).
Build quality is nearly to L standards (with the only downfall being that the front element apparently isn't weather sealed - for wet or dust conditions I use a UV filter, just in case). Focus and zoom are smooth and everything feels solid. Mine shipped from Australia to Canada - the box was really banged up after 10 days of travel though airports and customs. Consequently I made sure to test it thoroughly to be sure the OS, AF and IQ were all perfect. No problems - it appears to have taken a beating and still function as designed.
I use it primarily for wildlife and there are two things I have been surprised with: 1. the OS works so well that I use this lens handheld more than on a tripod. 2. I use the zoom more than anticipated (as I was considering the 300 2.8 it appears I made the right choice).