[KenCo]

I can't give you a detailed review as I don't feel I know enough about photography to do so but.....I own the 17-55 f2.8 is usm and had to think long and hard over this and the 17-40L usm. I shoot mainly landscapes but wanted to try more portraits and maybe even weddings and it's that, that swayed me in the end to buy the 17-55. I done a lot of reading and did find some who were not happy with the 17-40 and those not happy with the 17-55 only really complained about the price and lack of hood included in said price.
The build quality compared to the 17-40 is noticeable (apparently, as I don't and never have owned one...Going straight from the kit lens like yourself) However weather sealing is lacking but only useful if your camera has it too though I haven't noticed any dust issues as of yet. Also, the Image stabilization DOES work on tripod, as it is the new generation IS (Just quoting what i've read) I have IS switched on at all times (even when not needed) on both the 17-55 & 70-200. There are problems with flare, again though, I got as much flare in similar circumstances with the kit lens. Sharpness though is amazing, I was quite happy with the kit lens really but once you try a lens of this quality you will wonder how you got along with the kit lens.....Just amazing!
If you are considering going full frame at some point, this lens won't fit....EFS only! and based on that i'd choose the 17-40. If you'd like to shoot indoors/low light conditions.....then F2.8 And IS as well as an extra 15mm is the way to go.
I have never tried the 17-40 but am more than happy with what I have now and if I had to do it again I would.

A couple of reviews for you.

(*Mod comment* - Sorry Ken, links to competing sites are against Pixalo policy.)

I'm sure if you do a search on Google for comparison shots from both lenses may help.