I wouldn't reccomend the 17 - 40mm L to anyone despite owning one! Of all of the lenses which show Canons inability to compete in the FF wide angle market the 17 - 40mm is the prime example.
At 17mm wide open there is no measurable resolution at all in the corners. Shots of brick walls will show the detail of the bricks simply fade away to a red blur. There's significant distortion and issues with vignetting making this a lens which you can only get acceptable results from at f/8 onward.
I you want an ultra wide lens then I suggest you look at Sigmas 8 - 16mm which was rated as excellent by photozone, but is an EFs. The drawback with any of these lenses is that you cannot use filters with them.
If you want a wide angle for FF then I'm afraid you're going to have to choose a prime lens if you want decent performance, even the 16 - 35mm f/2.8 L isn't fantastic.
Roll on the day Canon learn how to make a wide angle for FF which performs as well as Nikons 14 - 24mm f/2.8 which you can actually use with an adaptor, as many have been forced to do.