I recently got the 50-150. First copy front focus horribly, visible even viewed on screen (without zooming in). Second copy is near perfect. Focal point is just a bit towards the back end of DoF but it's ever so slight and it helps to blur the background so I don't mind. I think most people who think their lens is soft at 150 2.8 really have focus issues. Even mine that front focused, I noticed that the subject's eyes would be soft but the ring on her finger would be incredibly sharp to use an example. The lens is very sharp at 150 2.8 *IF* if focuses correctly. I wish Sigma would get their quality control together.
You'll find samples from my 2nd copy here, all 100% crops (about half the image) taken at 150 2.8 (maybe a few at 100mm in there but I don't think so):http://www.advweb.com/tmp/50-150_2/2/
Also, the min focus distance is 1m but that's for 50mm. At 150mm it will front focus at 1m and that is "normal" for this lens (that's just how it is). I find at 1.5m the focus is fine. Sometimes I like to shoot real tightly cropped portraits so I have to watch out for that so I'm not too close.
You can see here a focus chart comparison of copy 1 vs copy 2:http://www.advweb.com/tmp/50-150_2/1/
however note that since I'm at the min focus distance (1m) the test is NOT valid. The first copy front focused on 100% of my portraits from any distance so there is no doubt it was defective.
If you want to try one, I strongly suggest ordering from a company with a very liberal return policy. Amazon is the best that I know of (and cheapest) with B&H being second. There are a few reviews at Amazon:
Returning mine was a breeze with amazon and I didn't have to pay shipping charges. Same is true with B&H but you get 14 days instead of 30 with Amazon...
Let me know if you have any questions about the lens.