Love the 100mm f/2.8! It is the best lens I own to show off the pixel depth of the 5DmkII, very sharp and plenty fast. AND... now that some folks just got to have the IS version, you can pick them up used in great (even mint) condition for less than $400 (most markets). Negotiate for the hood and and filters and you are looking at a substantial savings. It is not all about the L series.
-Shea