'Magician' by Raymond Feast. It's cobbled together from every other fantasy book you have ever read. I was dismayed when it showed up on the BBC's recent 100 favourite books poll. No wonder sci fi/fantasy gets a bad name.

Also, I agree with the point that some books are well worth reading once, but if you find the subject matter challenging its difficult to go there again. I've found this with several books, including 'The Black Album' by Hanif Kureshi and 'Go Tell it on the Mountain'