I received mine yesterday, but didn't get a chance to play with it a whole lot. As expected, the lens is big, and moreso, quite heavy. The front element is much more protected that I had expected and makes the TS-E 17 look like a foolish design. The lens cap is somewhat odd, and as Keith said, much lighter than expected. Mine doesn't want to snap on evenly on both sides, but that might just be me needing to get used to it. 11mm is insanely wide as expected, and I appreciate Keith's distortion tests. That seems to match what we're seeing in param's shots and is great news for architectural shots. Unfortunately, this is also making me consider a geared head.
I thought it might be a less practical, but still viable replacement for the 16-35 f/4 IS, considering I have the 24-70 f/2.8 II, but after handling the 11-24, I'm not so sure. The 16-35 is the perfect travel and walk around lens and 16-35mm range is really practical. The 11-24 range reminds me of the Sigma 12-24 I used to own. It's great for landscape and architecture, but the focal range is very limiting for other types of shooting. 24mm is still quite wide, whereas 35mm is a moderate focal length for general purpose work.
On the curved tree issue, when I was shooting at 12mm, I would either crop out the far sides, or get in close to the trees and use the curves to add to the perspective and drama of the shot. I hope to get out and shoot over the next few days and I'll try to take a few shots to show what I mean about that.