That's not a bad call actually, I'm assuming it's a fair bit bigger and heavier and f4 is slower than I'd like (no IS either) but I've just seen one for almost £100 less than the 24mm. I'll need to read some reviews, if it's anything like the other cheap L zoom (24-105mm f4 L IS USM) the edge sharpness, barrel distortion, vignetting and CA will vary a lot across the range. Still, 17mm might be fun!I saw EF-S in the comments, but got lost along the way!
How about the EF 17-40mm f/4L? You get an can pick them up new for $500 in the US. Probably about 20-30% off, if used.