Originally Posted by vachss
As an owner of plenty of L lenses I really disagree with this. The L designation is only used for full frame lenses, but the 17-55 EF-S has optics that are at least the equal of many L zooms. I've owned both the 17-55 and the 24-70L in the past and think that the EF-S is actually the sharper of the two. Certainly it has a more useful zoom range on crop bodies (and it has IS). I will agree that the build quality isn't of the "main battle tank" level used on the 24-70, but that's about the only area in which it suffers by comparison. Once you add the 17-55 into the mix the 17-40L becomes pretty superfluous as well.
I've often heard the argument that EF-S is worthless if you're ever planning to go FF. The problem is that if you have a crop body today then EF-S lenses may very well be your optimum choice today. Good lenses, L or EF-S, can always be sold without much loss. If you go FF, sell your EF-S lenses and buy the lenses you need then. Likely as not, though, even when you go FF you'll want to keep a crop body around for some things and may decide to keep your high quality EF-S lenses after all.
To the OP I'd ask what problem are you trying to solve? L lenses can be the right choice for some things, EF-S lenses for others and manual focus primes for still others. For high quality general purpose use I'd recommend the EF-S 17-55, for shooting in a war zone or inclement weather I'd recommend the 24-70 (and a 1-series body), for low light and shallow DOF subjects nothing beats fast primes.
the 17-55 is a lens that i've heard good things about, but i'd still prefer the 24-70L on a crop body.