Mock exams are like any other information on the web. They are only as good as the people who provide them. Anybody in the world can make up test questions, but that doesn't mean that the questions are good preparation for the real exam. The few people who really know what's on the exam have signed non-disclosure agreements with Sun. Those of us who have seen the questions and write books about the exam (like Kathy, or myself) have to be really careful not to reveal too much.
Of course I'm biased, but I would recommend getting a couple of good reliable study guides, and making sure you digest them. That way you know you're studying material that will really help you on the exam. Also, if you're in doubt about how something works, write small programs that will teach you the answers. Get your hands really dirty!
Consultant to SCJP team.<br />Co-designer of SCJD exam.<br />Co-author of "Complete Java 2 Certification Study Guide".<br />Author of "Ground-Up Java".