Man, I've debated this myself, quite a bit. I went with the 15-85mm, which generally matches the sharpness of the 17-55/2.8 while being 'better' in every category except low-light focusing and stopping.
Part of my decision was that I wanted more lenses; getting a tele-zoom and grabbing primes makes sense for me as a hobbyist. But if I was going to mount just one EF-S? It'd be the 17-55/2.8. I can zoom with my feet.
And Canon L-glass? That's hell, in every way, if you're not affluent/serious/professional. Crop sensor SLRs have a naturally limited FOV due to the flange distance that the mirror box imparts, so even your widest FF zooms are still going to be pretty narrow. They also don't make any L-glass that is fast, wide enough for APS-C, long enough for a walk-around, and has IS- and you're losing 63% of the light. L-glass is basically throwing money away for those using the smaller sensors and is not terribly worth it if you don't have a defined application that you know the camera and lens can handle.
But it's not all bad- with L glass, you will still take incredible pictures (ask JAE about his 100-400), it's just that you'll have to give up something. Mostly you give up IS, but you may give up the faster aperture as well, and you will be adding quite a bit of weight.
Personally I've stuck with non-L IS USM EF lenses and assume my 15-85mm will stay on my 60D if I ever grab a 6D or 5D, though my 24/2.8 IS USM is really the only nice EF lens I have.
Canon 6D|24-105/4L |70-300/4-5.6 IS USM|50/1.4 USM|50/2.5 CM|50/1.8 STM|85/1.8 USM|Samyang/Bower 14/2.8|24/2.8 IS|Sigma 150-600 | C
Canon EOS-M|11-22 IS|22/2|EF-M 18-55