If you need to go wider than 24 (38.5mm effective compared to 135 format) then I'd suggest either
Sigma 10-20mm f/4 - 5.6 EX DC HSM ~ £300 - if you're shooting landscapes
Canon EF-S 10-22mm f/3.5 - 4.5 USM ~ £500 - if you're shooting mixed subjects
Canon EF 16-35mm f/2.8L USM ~ £1000 - if you're shooting people
From all reports the Sigma is an excellent performer and clearly offers great value for money.
When you're down at the extremely wide end of things, the 2mm extra reach of the Canon 10-22mm can make a lot of difference. In 135 format terms, you're looking at the equivalent of 16-32mm for the Sigma and 16-35.2mm for the Canon... and those extra 3.2mm relates to a significant change in the lens' field of view.
Lastly, the Canon 16-35mm represents a good long-term investment... particularly if you're considering moving up to a FF dSLR anytime in the next few years. Whether you can justify such an investment is up to you
