I'm in a similar position. Whilst laying out huge sums of cash does make me a little twitchy - the fact that lenses hold their value well means I accept saving a little longer for a lens I want irrespective of price.
So far I've narrowed it down to Canon's 10-22, Tokina's 11-16 and Sigma's 8-16.
From what I read, the wide angle focal length makes a big difference. The Canon can show you 10% more than the Tokina and the Sigma 20% more than the Canon.
Tokina seem to have the fastest lens and arguably the sharpest - but also have the largest quantity of complaints about CA and flare. It comes with a hood and is re-assuringly weighty.
The Canon is, as per usual, berated for not coming with a lens hood which seems to be mandatory for lenses with such high viewing angles as the front element is more exposed to stray beams. Apart from that, I borrowed some content from a user on pbase.com;
Resolution (center): Tokie gets the slight edge here.
Resolution (edge): Tokie is the definitive winner here.
Contrast: Slight edge to the Canon, if pressed on it.
Shading: Tokie the much stronger performer.
Distortion (barrel): Canon by a little.
Flare: Canon by a lot.
CA: Very close, but the nod to the Canon. Both "so-so".
Build: Tokie, though the Canon is still a solid lens.
Handling: Nod to the Tokie because the focus/zoom rings are similar in size and the dampening buttery smooth.
Aperture (speed): Tokie, of course.
AF (noise): Canon takes this one.
AF (speed): Canon again, though the Tokie is a very close second.
MFD & Magnification: Canon here too.
10D/D60/D30 compatible: Tokie yes, Canon no.
Talking about the Sigma, it's another one to two hundred quid on top of the other lenses, but it seems to have the least barrel distortion (fisheye effect) of the three, despite its extra 2mm over the Canon.
In my personal opinion, I'm thinking that if I buy the Canon, then I get some sort of peace of mind that I wouldn't with the Sigma. I fear it's a placebo effect though and that one of the others are better! Aaargh!