Sigma 18-50 2.8 EX-> EF-S 17-55 2.8 IS worth it?

Planning on upgrading from a 20D to a 7D for improved focusing and video capability. I currently have a Sigma 18-50mm f2.8 EX DC lens that I've been pretty happy with. It is tack sharp in the center even wide open and across the image at f4 and above. Its shortcomings are Chromatic Aberration and a bit soft in the corners at 2.8. I know 'worth it' is subjective to one's finances, but I'd like opinions on the improvements I'd expect from upgrading to the 17-55 f2.8 IS lens for both still and video use (i.e. how useful is IS in this range on a 7D for still and video).