Used two study guides, K&B 7 and Mala Gupta's book. I like them both and they work well together by providing slightly different approaches and explanations of the certification objective topics.
Like everyone else, I thought the enthuware mock exams were a great value, but they were different than the actual exam. They contained questions on topics that are outside the scope of the exam (I remember several questions on arraycopy in particular), and they have some true/false questions while my actual exam had none. Still a great bargain and good for preparing for the exam experience.
I read Roel's exam report in which he finished quickly and was able to review all 70 questions. It took me about 95 minutes to complete the 70 questions, and I was able to review 30 of them - I changed 4 of the answers of those 30 so I think I would have scored higher with more time to review. Time pressure is a factor. I have seen others mention that it seemed as if many of the questions at the beginning of the exam were more difficult than those that came later. I didn't think that was necessarily true, but some of the questions had a lot of code to work through.
I read another exam report that suggested a quick glance at the answers to see if "Compilation Fails" or "Runtime Exception" are among the choices, which I thought was good advice. If those are not there, then I would start at the main method and work through the code to come up with an answer, then see if my answer was one of the possible answers. If they were there, I'd spend 10-15 seconds first to see if I could spot an obvious error before working through the code.
I have Oracle DBA certifications, but this is my first Java cert. Like the DBA certs, I find the value in obtaining the cert is that it requires you to have a comprehensive knowledge of the certification subject. If you have learned your DBA or programming skills on your own, you will have learned just what you needed to know, not all that there is to know. Studying to pass a certification test forces you to learn more.
Now on to OCPJP