There was a Mock
Test at IBM's site (named ICE, if i am correct), which was the toughest when i took the test (may 2000). it was very slow (net speed) but was certainly the best exam.
sorry, i don't have the link or don't know if they still provide that free exam...but just search once.
also just to mention I got 64% in ICE while i used to get almost 100% in other exams (Marcus and others).