I've tested it briefly on a 10D... it's not so good. Decent if you're looking for something other than a 28-80mm kit lens to hang on a cheap film body, but pretty soft when compared to good glass.
IMHO it is not good practice to buy a $3,000 camera and put a $200 lens on the front of it. Spend another grand for some L glass, or sell the 5D and downgrade to a 20D if you've run out of money.
The 5D has ZERO advantage over the D60, 10D or 20D if you are using a 28-105mm lens on it and decent glass on the others.