Dave Dugdale from Learning DSLR Video has reviewed quite a few products on his website, and if you're in the market for a super-wide lens on full-frame or a very wide lens on APS-C without going into fisheye territory, his latest review might be right up your alley. In the video below, Dave takes a look at the Canon 16-35mm f/2.8L II and the Tokina 16-28mm f/2.8 on the Canon 5D Mark III:
It's actually surprising how sharp the Tokina is, especially considering that the Canon is $1,000 more expensive than the Tokina at $1,700 vs. $700. I tend to be allergic to focal lengths anywhere below 22-24mm for cameras like the Mark II/Mark III, but then again I'm not much of a landscape shooter in video or photography. Both of these lenses should also be good options if you're using APS-C cameras, as they go wide to about normal focal lengths. While you might have your eye on something crazy fast like the new Sigma 18-35mm f/1.8, there may be situations where you want that extra 2mm or you need it to work on a larger sensor.
If you'd like a little more detailed information, head on over to Dave's site.