I passed
SCJP about 3 months ago. I used the JQPlus to prepare for the exam and found that the JQPlus was harder than the actual exam. Thus, I was scoring around 80% for the JQPlus and 100% for the Marcus Green's exam but I got 92% on the actual exam. So that was my experience with the mock exams. I have never tried Jwhiz but I think it should be very similar to JQPlus.
To get over the hump... For the questions that you are getting wrong in the exam, I would copy all of the detailed explanations for the answer into
Word. And then learn all the explanations and also look at any related API. In addition, note the reason why you missed the question - whether it was a careless mistake or something tricky that you missed. I had a separate
Word file for all the tricky things to avoid - like uninitialized local variables etc.
That's how I prepared. Hope this helps.