This is attracting a lot of attention at the moment, reflecting the general disatisfaction with Canon wide angle lenses. The most popular solution is to use Contax/Zeiss wide angles in either the MM or AE mounts with an adapter. There's plenty of adapters available, but the most popular quality adapter comes from Cameraquest.
I've been considering this for a while but rather than pay serious money for a 21mm Contax, which is the most highly regarded Contax lens, I decided to try out the Contax route with a less expensive 25mm. I evaluated it against my Canon 24mm T&S and 24-105 4.0L IS. There isn't much betwen them, especially as I almost always use wide angles at f8 or f11. I suspect that the only Contax lens that really makes sense is the 21mm, but that's about four or five times the price of any other Contax wide lens!