Do you want a fisheye lens or a linear wide-angle lens? The 8-15L and the Samyang are both fisheye lenses. The 10-22 is not.
If you want linear wide angle and you're on a budget then why not have a look at the Sigma 10-20mm? I've owned both the Sigma and the Canon 10-22 and while the Canon is better, it was not so much better that it was worth paying the extra in my opinion, if you can pick up a secondhand 10-22 for the price of a Sigma 10-20 then I'd say go for it.
Keep in mind though that the 10-22 and the 10-20 are both designed for APS-C so you won't be able to carry them over if you ever decide to upgrade to full frame.
If you want the option to carry the lens over if you move to full frame then have a look at the Sigma 12-24mm not quite as wide on APS-C but super wide on FF and it's a really nice lens.
Look up reviews: http://www.photozone.de/Reviews/overview
Then look up sample images for each lens and decide which would best suit you:
If you have a crop-sensor camera, Canon's 10-22mm is definitely the way to go.
@stenchlord: Thanks for the recommendations.
I picked a up a second hand Sigma 10-20 really cheap and I'm super stoked with it!
@oldmanlongstaff: I'm glad, I look forward to seeing your photos in the photo thread :D