The EF-S 17-55 f/2.8 is my favorite lens and is attached to my camera much more than any other lens. The image quality is similar to the 24-105 L you have, if slightly higher. Check out the MTF tables at the links below:

http://www.photozone.de/canon-eos/42...is_50d?start=1

http://www.photozone.de/canon-eos/42..._4_50d?start=1

I really like having a crop sensor camera. You get a much larger selection of lenses, and on regular EF lenses you only use the center of the lens which is almost always the sharpest with least distortion. I also really like the 1.6 times more reach you get. I started with a 20D which I upgraded to the 7D a few years ago and absolutely do not regret getting another crop sensor camera. I don't do much photography in dark conditions without a tripod, so for me the 7D does everything the 5D I/II/III can do and more.