good performerCanon 10-18mm is a great little lens for not much money, actually preferred it over the more expensive 10-22.
Good article, really interesting, should be of interest to the OP, will be interesting to see what he wants to shoot.A lot of what uncle Ken says can be ignored but this is actually a good article:
http://www.kenrockwell.com/tech/how-to-use-ultra-wide-lenses.htm
I would read it before deciding if you REALLY want in UWA lens.
But not much use to the OP who has a crop sensor camera, and already has a zoom that goes to 18mm.My preference is for the Canon 17-40L .. very versatile great results and can be had now for a good price