I've been looking to upgrade my 18-55 non IS lens to the IS version because I found it disappointing. Instead I stumbled upon a chance to buy a Sigma 18-50mm f2.8 EX DC lens and my first impressions are that the difference in the two is night and day.
Can't speak for the IS version though. Would be interested to know if anyone has compared the non-OS Sigma with the IS Canon lens????