Hi Rui Min,
The best places to look for this is:
1) Official OCMJD page
2) Monkhouse's book for SCJD (yeah, it is SCJD, not OCMJD, and it is for JSE 5, not JSE 6 or 7, but that doesn't make any difference. The book is extremely useful)
3) This forum
4) Roberto Perillo's SCJD paper
Now coming to the advantages of this certification:
1) I assume you are aware that you have to clear any version of
SCJP or OCPJP to be eligible for this exam. So, in any probability, this will be your(at least) second certification, which shows that you are quite serious about Java.
2) Unlike SCJP/OCPJP, this certification does not simply relies on theoretical concepts. This certification shows that you understand basics of Java, and can independently handle a small project on your own, and you can write a code which works in networking environment, and you can write thread-safe code et-cetera.
3) If you like coding on your own, you are gonna enjoy this certification. Its real fun in reading the requirement document(i.e. assignment you'll get after you purchase the certification), thinking about how to implement a part, thinking about which design decision to take(and how to justify it during essay exam), making choices about where to put efforts and where not to put and so on.
For me, this is the certification I enjoyed the most out of all my SUN/Oracle certifications. However, please note that if your daily work is not in Core Java (and is in
JEE etc.), then from technical perspective, this certification will not help you much during your work. However, its a nice experience to handle a project on your own.
I hope this helps.