I've got the 5DII and have used the 16-35mm II for a couple of years & do like it. Mine's just a hair soft on the left side unless I stop it down to f16 or so, but it's super sharp in that range. I've heard that several have that issue.
I bought the Nikon 14-24 this past winter & got this adapter for it: http://www.adorama.com/CZNKGEOSAF.html
I think I wasted my money on getting this more sophisticated adapter. The one without the added chip would have been just as useful. Supposedly this chip enables the focusing confirmation function but it didn't work on my copy. The lens is stunningly sharp wide open & stopped down in the middle and the corners. I get the extra 2mm of wide angle - you can almost see your ears in the viewfinder
I shoot landscapes only, so I'm always manually focusing whatever scene I shoot, so that is not a big deal for me. If I was using it to wander around & shoot, I would not buy that lens given the manual focus.
Three other items to note:
1) you cannot use filters on the front of the 14-24 lens unless you buy some pretty exotic and expensive adapters - I have not bothered with this - so if you want a polarizer, forget about it.
2) the aperature of the lens is adjusted with a manual lever that protrudes from the adapter - you slide it one way to open up, and the other way to stop it down. There are no marks or clicks to let you know where you are between F2.8 & F22 - it's a guess. It's not hard to get it in the ballpark of what you need though. It just takes some getting used to.
3) You have to open the aperature wide open to focus (manually), then stop down to the desired point to shoot the image - stopping down significantly reduces the light in the Live View, or view finder & it's tough to focus that way.
If you can live with these caveats, its a fabulous lens, even on the Canon body.