Hi,
The NBME exams are great for several reasons (no, they don't give you the answers. Just your score and score distribution by subject):
1) It has predictive value. Often times what you get on the NBME (esp 2-4) will be very close to what you will get on the actual exam.
2) Its a great way to gauge your progress during your prep. Ideally, you want to see an upward trend in your scores. (210, 225, 238, 244, etc)
3) And they're great as a learning tool. But how? They don't even give you the answers. Well my friend according to some people on SDN, the answers are floating around somewhere on the internet. Don't ask me for more info about this. Know what I'm sayin?
So you take each test in TUTOR mode (gives you 4 hours per block of 50 questions). You spend 1 hours doing the 50 questions. Then you go back to question #1 and you can spend upto 3 hours going over each question in great detail. Figure out what its asking. What's the CONCEPT. Figure out the correct answer (don't change your original answer at this point! You'll falsely elevate your score prediction). Then figure out why the wrong answer is wrong. If you see graphs make sure you understand them well. Often times you will see the SAME EXACT graph on the actual exam!
I promise you if you use this approach will learn a great deal. Remember these are ACTUAL Step 1 questions. Nothing compares (not even UW). Read peoples post-exam reviews. Most people agree that their exam was most similar to the NBMEs in content and difficulty.
G'luck!