Well worth it! I do a lot of book buying and selling there and so far never experienced a bad trade. (but maybe book folk are just that bit more honest :) ) Really good for getting moderately rare stuff without paying astronomical prices and if you miss one now there'll be another along in a day or so.

The only real drawback is being sniped (having someone outbid you at the last minute) but even that doesn't happen often. If you know what your bidding limit is, ebay will handle all the bidding for you.