I took the exam today and it was easier than I thought.
I got 120 questions and 2.5 hours to finish it. I answered all questions and at the end I had about 15 min left to review them. Some questions took me more than a minute since they required to mentally run the program or watch carefully for compilation or inheritance problems.
I didn't prepare for the exam (altough I already knew about lambdas and a little about the new Date/Time API). I think anyone who has some years of experience in Java or had already taken the Java 6/7 certification can confidently take the exam. If you feel you need to study, any OCAJ 7 guide will do the job. In fact, I only got 2 questions about lambdas (one about a simple expression and another one using the Predicate interface) and 3-4 questions about the new Date/Time API. As you can see in this FAQ page, the exam is almost identical to the previous version. If you don't know about these topics, the following two articles are what you need for the exam (well, according to my experience):
Exam results will appear in about 8 weeks. For the reduced price, I think this is a great opportunity if you want to update your Java certification or to take this exam instead of the Java 7 version (if you don't mind waiting for the results and/or sitting for a longer exam) .