I got the 17-40mm....I found a good used one for like $500 I think and figure that would hold me till I went for a 16-35 later.
But honestly, I love the lens, this and my 24-105 are usually my daytime walk around lenses.
I actually wanted something even more UWA to play with, and got the rokinon 14mm...which is about the last on the edge of rectilinear.
that thing is WIIIIIDE...but has proven for something relatively inexpensive (I caught a special sale for like $199 I think?)....it is a LOT of fun to play with. And for $25 I found a program to correct for the problems the lens is knowns to have. But I've had a LOT of fun playing with this UWA lens, and the 17-40mm, is quite easy to walk around with and use.
My next lens..I'm debating between the 24mm tilt shift, or a 50mm L f 1.2.....I'm leaning towards the 24mm TS here lately, I like the wide angle stuff.