Canon EF 17-40mm f/4L USM Ultra Wide Angle Zoom Lens

Pros: Sharpness (at f/8 or above)
Excellent Build Quality
Non-rotating front lens element (Makes using polarizing filters MUCH easier)
Smooooooth Focus and Zoom rings

Cons: A tad bit expensive, but worth it.
Kind of funky looking lens hood.
f/4 means low light conditions require flash

Summary: This lens was my first "L" lens, and I made a great choice. With aperture set to f/8 or above, you can capture some pretty incredible shots. I recently shot some pictures of pumpkins and they practically pop right out of the picture. (I also did notice a surprising number of other folks using DSLRs, but none of em had L lenses, and I caught a few glances of people checking this lens out).
I'll be honest though. When I first got this lens, I was a little dissapointed because it didn't really seem to be that much better than the kit lens that came with my Digital Rebel. After using it for a few months now, though, I've come to realize just how wonderful this lens is, and it rarely leaves my camera. The only real con is that f/4 means you'll need a high ISO setting or a flash indoors. This isn't a problem so much with higher end cameras with better ISO processing, but with consumer end DSLRs it may be a bit problematic. Although odds are, if you're looking at this lens, you're probably not using a cheap camera to begin with!