Useful for timing but that is about it. Some of the topics were similar, but I thought there was less crossover between NBMEs and Step 2 than Step 1.
I did NBME 3 and 4. They both underestimated my score, but 4 was much closer.
Edit: The general consensus on this board is that UWSA is the best estimator. Quite a few people claim it comes within a few points of your actual score. For me it came much closer than NBME 3 but not as close as 4.
My school recently had us do one of the Clinical Science forms. It seemed rather formal as they simulated real testing situation and didnt give us our results for 3 weeks.
All in all it was not very hard if you've gone through some study sources (namely Kaplan, First Aid, Step Up) and done a few questions. I'm afraid to say it may not be indicative of the real test! (Though I have no real basis for this answer)