Thanks go out to all the study guide and the official Java tutorials authors and the good folks here at JavaRanch.
Here's what worked for me :
Preparation material: The Sierra and Bates book
, the Sierra, Bates Practice Exams book
, the Mughal and Rasmussen book
and the excellent Official Java SE tutorials
. Sierra and Bates is the more readable and "fun" book. If you are buying just one book - this would be my choice. However, some of the topics are covered sparingly, for e.g. Enums and Generics. (Note: I had the older Sierra/Bates Java 5 Exam Guide). I understand that the authors are trying to focus on the content only from the examination point of view, but in case you feel that you need in depth coverage - then Mughal,Rasmussen is your bet. This book delves into all topics comprehensively and sometimes focuses on the API level stuff, the writing style is dry, but overall a great book to supplement Sierra,Bates. I would also highly recommend the official Java tutorials in case you don't find the books good enough. The Regular Expressions tutorial is especially good.
I went through Sierra,Bates completely more than once and used the Mughal book only for the topics I wanted more practice on. I made notes for all things that I felt were important to remember or were tricky. In the week leading up to the exam, I more or less looked only at my notes for revision.
If you are interested, they are available at : http://suhrid.net/wiki/index.php?title=Category:OCPJP
I also wrote a lot of practice code using the Eclipse IDE. Sierra, Bates do not like using IDE's - but I found it very useful. If not for Eclipse, I dont think I would have written as much of practice code as I did. Eclipse's continuous compilation means you can keep experimenting without wasting time on the command-line. Of course the command-line javac/java/jar, classpaths etc needs to be practiced outside the IDE.
I took all the exams in the separate Sierra, Bates Practice Exams book. I would highly highly recommend that you take all these exams and analyze your performance. I averaged between 70-80% in these exams. The questions are slightly tougher than those in the real exam - but they do a fantastic job in preparing you for the real thing. I also did the full mock exam in the Mughal,Rasmussen book. Finally, I did an online mock which Pearson offered me when I booked the OCPJP exam. If you get a chance, do take this - because it mimics the real exam interface closely and the difficulty of the questions are somewhat similar to the real exam.
The Real Exam:
The real exam covers all the exam objectives well, the questions are well designed and the exam interface is excellent. Many exam takers have said that the questions are "easier" than those in the mock exam. I would say that yes, while some questions are a little easier, on the whole the questions do a good job of thoroughly testing your knowledge of the exams objectives and I did find some tricky ones in there as well.
Make sure you study all the objectives well, code a lot, take notes, take the mocks and analyze them and you'll be certified in no time.