I've used my 16-35IIL so much, it's one of my heavy use lenses. It's the only lens I've ever had to replace the front element on. I've had a number of ultra wides over the years and the 16-35IIL is the best of the lot. While it's not a modern lens and could do with some serious upgrading, it's still the most versatile ultra wide available on the Canon mount.
Tamron 17-35 f2.8-4 dii. A sharp lens, but flare control was apalling and the distortion was the worst I've ever seen in a lens.
Sigma 12-24 HSM, an odd lens. Hard to put filters and mine has awful corners with any aperture bigger than f11....but that angle of view is amazing and it's the most corrected lens for angular distortion I've ever seen.
Canon 17-40L, it's a lovely lens. Relativly cheap, sharp enough and very versatile. I only sold mine because I needed the f2.8 aperture.
Canon ef 16-35IIL, while far from perfect it's the most versatile ultra wide lens there is. Great colours and contrast, nice sunstars, epic build.
Once upon a time ultra wides were the re-mit of pro photographers and considered a niche lens. These days, everyone's got one...but one questions why? Many sit on shelves gathering dust because the 24mm end on most zooms is usually wide enough for most uses. I think this lens get a lot of unfair critisim. My advise is to stop looking for optical perfection and get out there and use it.