I have been debating a super-wide for a while, waffling between buying a new body to accommodate a Canon 10-24, the Tokina 12-24, the Tamron 11-18, and the Sigma 12-24/10-20. I have heard nothing but good about the Canon lens, but it's quite expensive and would require me to buy a new body. I have never been a fan of Sigma, and their build quality always leaves me cold. I have to admit I kind of forgot about the Tammy. I prefer a fixed aperture lens, one that focuses internally, and one that stays the same size while zooming. The Tokina came out as my favorite, in fact it's the only one that offers constant aperture. I took it out on a jaunt today.
The lens feels really nice, it's well built, heavy(I like heavy), it has a nice crinkle finish, and over all just says quality to me. I thought I would like the center pinch lens cap, it certainly makes pulling it off easier with the hood on, but I have trouble holding on to the ears and nearly dropped it several times. Maybe I'll get used to it, just a niggle to me though. The focus is accurate albeit slow, it uses a screw drive, and it's just not close to the same league as my USM motors, or even my old 80-200 f/2.8's advanced arc form motor.
I found myself composing at about 20mm or beyond. My 17-40 was my widest lens, and I just found myself composing for this range. I had to purposely force myself closer to my subject to use the wide end of this lens. So far I like what I see. The lens is sharp, focuses fast enough for my use, and has a great range(that I have to get used to using). I am thinking my 17-40 might go away, and a 24-70, 28-75, or 28-70 f/2.8 may be in the queue. I need to do more testing on this lens but so far I like what I see.