I got this lens because I wanted a super wide-angle lens but couldn't justify one of Canon's fancy L lenses. I'd always been a bit wary of 3rd-party lenses, but I read a few good reviews so thought it was worth a go. I'm pleased that I did - this is a really high-quality lens.
The feel is as good as some Canon L-series lenses - comparable to the Canon 24-70 L lens I have, but much lighter. It's certainly way better than the standard Canon lenses in the same price-range, which feel plasticky and insubstantial in comparison. The foocusing and zoom rings are smooth and solid, without any give or rattle. The image quality is also great: Sharp and pretty good contrast. Even the lens cap is better than the standard Canon ones! (It uses an easy-grip system so you can get to it even with the hood on). You do get some distortion at really wide angles, but then, that's part of the wide-angle look.
Although it is designed for APS-C type sensors (1.6 crop), I have also used it on a full-frame 5D and it's fine. You can go as wide as 16mm without vignetting and it gives *really* wide-angle shots. Beyond that it just starts looking very 60's, which is actually quite cool.
The only unusual thing about the lens is the way to switch between manual and automatic focus; You push the entire focus ring backwards or forwards. But even that is arguably an improvement on the standard small switch, as your hand is automatically in the right place, so no searching around for a fiddly switch while you're trying to compose your shots.
Good:
* Amazing build
* Very good image quality
* Fast focusing
* Lighter than the equivalent L-series lens
* Cheaper than the equivalent L-series lens
Bad:
* Um. Would like a larger max-aperture (but then I always want that).
* Nothing?!
On the strength of this lens alone, I would be happy to buy more Tokina lenses.