The MyBooks get my vote too! I've got 3 and I got my parents a 500GB for TimeMachine on their iMac.
I've got two 500GB units and a 2x500GB dual disk. One of the 500GBs is attached to my Mac mini HTPC and the other units are connected to my iMac, the 500GB is for TimeMachine and the dual disk runs as 500GB mirror providing protected storage for my Lightroom and iTunes libraries....