WhizLabs contains 6 full mock exams plus 150 quiz questions. After write the diagnostic exam, which is tougher than real exam, WhizLabs' reports give you a large idea where you have to make efforts in order to archieve the SCJP and win your weaknesses.
I also cannot forget about WhizLabs refund policy : If even after purchase WhizLabs simulator you still cannot pass the real exam, they are engaged to refund you all amount you spent.
"If someone asks you to do something you don't know how to, don't tell I don't know, tell I can learn instead." - Myself
I'm currently using it and find it to be pretty helpful. I like how the UI is designed but there are a few issues. Sometimes I'm not able to read the drag-and-drop questions. But overall it seems pretty good. And as Edisandro mentioned, you can get a full refund if you fail (I didn't read the details since I'm planning to pass ).
You can also download the free demo so you can see what the questions look like. Good luck.