I am very happy with my Canon 10-22mm EF-S. I had seriously considered the Tokina 11-16mm 2.8, but decided to try the Canon first. I find the 10-22mm very handy for crime scenes, teamed with a 35L and Macro 100L. The 10-22mm can also be a fun walking-around lens, as the sole lens for the day, though it requires a mental adjustment if one is accustomed to longer focal lengths.
Though the 10-22mm's aperture cannot open as wide as its Tokina competitor's, longer exposures on a tripod make a wide aperture less needed, and landscapes and architecture would, for most folks, be shot stopped-down quite a bit, anyway, to get deeper DOF. (I am speaking from the point of view of crime scene photography, which incorporates elements of landscape and architecture photography.) I often do shoot with my prime lenses at wide apertures, but do not seem to find myself wishing for that capability when shooting ultra-wide.
Having said that, I keep reading and hearing good things about the Tokina, and may well try one, someday. To be clear, I do not consider myself an authority; more like a serious beginner.
Canon 7D/5D/40D/1D2N; Nikon F6, FM3A, & Coolpix A; Canon 40mm Pancake, 135L, 50L, 35L, pre-II 50mm 1.8, 100mm 2.8L Macro, 10-22mm EF-S, 28-135 EF, 400mm 5.6L; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S; Tokina 17mm & 100mm 2.8 Macro