I got mine about a month ago and am just back from 10 days in Italy, where I used this extensively.
Sharp, good flare control, lots of fun. The previous reviewer says something about the AF being slow-- which, while true, is only half the story. You're almost always going to be focusing at or near infinity, so it's actually very rare to have to wait. Because this lens has such wide depth of field, I actually usually keep it on manual focus and leave it near infinity.
Actually, the biggest challenge with this lens is composition-- it can be surprisingly hard to find a scene with interesting things in all directions. That said, this lens is *excellent* for inside buildings and crowded city streets. Be aware that outdoors, finding the correct exposure can be difficult for a number of reasons. I did significantly better in M (manual) mode than my camera was doing in A (aperture) mode. (reasons for difficulty include the dark corners in the frame which mess up the camera's metering in some modes, and the probability that the sun will be in the frame and/or there will be both very light and very dark areas in the frame)
This lens is excellent for making spherical panoramas, and for realllllly wide angle shots (I'm de-fishing a lot of my pictures with software).
(I use this lens with a Canon 10D)