I can't comment on how much they resemble the real MCAT, because I haven't taken in yet, but they have definetly helped me to target my studying. I starting taking practice exams 3 months out and I will take the June 13th test. The tests are supposed to be compiled of retired MCAT questions, so they should be a pretty good indicator of how you are doing.
My scores have been as follows:
Exam3: 24
Exam4: 29
Exam5: 27
Exam6: 29
Exam7: 30
Exam8: 31
Exam9: 32
With the exception of exam #4, I have had a nice steady increase in my score as I have studied. And for the record, when I finished exam four I was very happy to get a high score so early in the study period, but at the same time, I knew it was a total fluke. In contrast, I'm feeling pretty good about that 32 I got this past weekend. It was earned. I'm aiming for a 33 or 34 on the real test. I don't know if I will get there, but I'm confident I can at least get a 30.
My advice is to use the exams to get a feel for your best test taking stratagy and to use the detailed analysis at the end of each exam to show you what subjects to hit hard in the coming week.