Well, I took SJCP and I began SJCD. I guess if you ever programmed in
Java, many of the topics on the first
test are easy. You just need to study some less used subjects. Also, the questions are pretty straight forward: right or wrong.
SJCD in other hand is more complicated. If you already work with the technologies required by the exam (locks, threads, rmi or sockets, swing) it is easier. Otherwise, you will need to "master" them. Also, you need to document everything and most problems require analisys and judgment (and documentation of your decisions), so everthing is a doubt.