I have the lens and I really enjoy it on a crop camera. It was my first purchase and it was basically bolted on to my camera for a long time.
That said, you are asking about full-frame. I did use it on a 5dII for a few days and I liked it there, too. I didn't really push it in terms of usage, but I could sit at the dinner table and snap three people across from me on the other side. I never had a chance to use it on landscapes.
If I was considering buying it new for a 5dIII, I think I might just opt for the 17-40. The slightly higher cost would be worth it to me in terms of build quality and zoom flexibility, and the loss of a stop would not matter given the higher ISO capabilities of the newer full frame cameras for shooting in lower light. I don't think you can even compare the 20mm to the 16-35, though. If you are seriously considering the 16-35, i think the prime would pale in comparison. However, if you are on a budget, love primes, shoot handheld, shoot wide open, or have a potential use on a crop camera, you'll enjoy this lens. When I update to fullframe, I plan on keeping the 20mm long enough to really push it before I choose to try an upgrade.
I used to tell people that it was a little prone to flair but I may have had a bad experience with a cheap/dirty UV filter. However, I did opt for the Canon bayonet hood and I think it helps.