I'm reviewing this lens with the previous reviews in mind. That saying, I wont bother you with the regular details.
I shoot weddings and live music, so a 2.8 or faster lens is a necessity. That was the main selling point for me. I've always liked sigma's build quality in relation to price.
I've also heard horror stories about sigma's quality control at the factory, but I've never had a problem with their stuff, and this is no exception.
I've never encountered the back focusing in real life. I dont shoot flowers close up or anything, so it's not a big deal to me. Many of the shots I take are at infinity anyway.
To combat the back focusing issue, I'll post direct tests. These are wide open at the max focal lengths.
At 50mm 2.8: Almost no backfocusing. The line is the main focus point, and it's on the front edge of the depth of field. Not really a big deal.
<a href="http://img.photobucket.com/albums/v74/nualln/lens%20test/sigmaat50mm.jpg">Focusing at 50mm</a>
At 18mm it's a worse, but not terrible. Give this sample a little more slack because I was shooting far away.
<a href="http://img.photobucket.com/albums/v74/nualln/lens%20test/sigmaat18mm.jpg">Focusing at 18mm</a>
Bottom line: This lens is pretty sharp at 2.8, and I dont have any problems with the focusing. Yes, there is a little back focus, but I dont regularly shoot pieces of paper with parallel lines. The effect is worse closer to the subject, but when I'm that close to something, I usually make sure it's in focus first. Be a smart photographer and think before you shoot. This lens is great.