Canon TS-E 24mm f/3.5L original
I use this lens for commissioned architectural photography, usually mounted to a 5D2. I've found it to be an excellent lens, but certainly not a perfect one. As the other poster said, the CA can be a problem at times but corrects pretty well with modern RAW software (I use CS 6).
It's not the sharpest lens in my bag but it's sharp enough, and I've made 20x30" prints from it that look great. For achitectural photography the shift movements and lack of distortion make up for any other shortcomings it may have. If you can get one for $600 I would call it a bargain.