If things seem too good to be true, they often are.

The Sigma 10-20mm f4-5.6 EX is a great lens, at a great price, but Sigma's quality control is letting out quite a few faulty lenses. The faulty 10-20mm lenses all have one trait, a very soft right hand side. This is due to decentralistaion, and will be apparant at all shooting conditions and all apertures.

As this is an EX lens, I suggest you take yours back if your copy shows noticable variations in sharpness across the frame. Take a look at the photos below. We can expect the centre of the lens to be sharper, but at f8 on a 1.6x sensor camera? Needless to say I returned both copies for a refund. The store manager (Jessops in Derby) said he had never seen this issue *cough* and promised to order in a batch for me to test.

I will fill you in on the details when I have tried a few more copies and see if I can get my hands on an elusive sharp one. The images below are 100% crops of the left, centre and right of a picture taken at 10mm, 125th/sec.