Just as a heads up, I took the free 120 at the Prometric center today. I thought it was really useful, though the first block was quite tough which was a bit demoralizing. I came home to check my answers and it looks like the test they administer at the centers is somewhat different from the one you can access online. I counted 46 different questions in all.
Some of them are completely new, others are expansions on some of the questions currently posted. For example, a question in the online 120 might ask: what is the Absolute Risk Reduction of blah blah blah... while the one at the Prometric center (same prompt) was: what is the annual cost of treatment in order for one person to benefit assuming a cost of $$$ per dose.... which would have you calculate the ARR, then NNT, then do a little math. I'm not sure if this trend holds true for all the questions that were different, just a few that I remembered wanting to review.
Overall I got an 88%, which hopefully has me scraping around the 250 marker. I made a few dumb mistakes, but there is some basic stuff I can improve on. Frustratingly though, I can't find the answer to 6 of the questions I missed because they aren't on the online version.