I've yet to see a review for the Sigma 10-20mm F3.5 EX DC HSM so I guess we'll have to rely on individual feedback on the Net for a while.
But I'd have to ask whether the Tokina AF 11-16mm f/2.8 AT-X Pro DX might not represent a better option than the Sigma f/3.5. It's brighter, at f/2.8, and fits in well with most APS-C kit zooms and, here in the UK so far as I can tell, it's cheaper. The PhotoZone review
states that it "is currently the best ultra-wide angle zoom lens for Canon EOS APS-C DSLRs
". Probably not the nicest thing I could have quoted as we are in the Sigma section (sorry) and in fairness they haven't tested the new Sigma.
As you've got another thread going here
I'll lock this one so that answers to your purchase decision can be kept in one place. c.f. the forum rules
section Got a question?