A friend had one in the Canon mount, and reckoned it was as good as their L-series lenses. They ended up getting rid of it, only because they went to a 5D Mk III (full-frame).
Other options would be: Tokina 12-28mm f/4 (£530 - I have the 12-24mm and it's good, so the 12-28 should be good, too), Nikon 10-24mm f/3.5-4.5 (£650), and possibly Sigma 10-20mm f/4-5.6 (£350).