Generally, in my experience, if you read the book first the film tends to suck. Prime examples are:
American Psycho (Pretty much un-filmable, due to content.).
Bonfire of the Vanities.
Clockers.
25th Hour.
An exception to the rule, however, is Charles Dickens' Bleak House. Reading that in high school was a most unpleasant experience, whilst the BBC's version last year was brilliant T.V.