I think Whizlab is a very sensible investment if you want to pass
SCJP 5. It has a very good interface that reflects the exam proper - including drag n' drop questions. You can also review all the exams you took, checking each question, comparing your answer to the correct answer and then look at the explanation. Altogether I think it's indispensable if you want to pass. I started off with some low scores but they gradually improved.
I took a couple LearnKey/MasterExams because they were free with the 'SCJP 5 study guide' (Bates & Sierra), the questions were OK - very similar to the book - but the interface was NOT good.
PS I passed the SCJP 5 exam today so it must have worked