I use to have the 10-20 on my crop, it was a really good lens for the wider stuff, some interesting images from it although oddly not many landscapes I liked.
Phil - I truly believe it was me rather than the lens - for oddball closeups it was great, theres a couple of my daughter in make up which are really nice - but for landscape everything just seemed to be sort of blugh.. I have my 10-20 gallery here if you're curious.
A lot of positives there for the 17-40 - guess you all disagree with this review (just 3 stars).
I use to have the 10-20 on my crop, it was a really good lens for the wider stuff, some interesting images from it although oddly not many landscapes I liked.
photozone.de review is spot on. However if you were ok with sigma 10-20, then you have nothing to lose. 17-40 is a bit better