I agree with the 24mm rule. I shoot with a 24-105 on my sister's 5d and at 24mm it's a great focal length. But I don't think anything more is really wide angle persay. A 24mm on my 1d isn't very wide, and I CONSTANTLY find myself wishing I had a wider lens.

AND after looking at all these photos I have decided an ultrawide will be one of my first purchases when i return to the states.