I took the onsite practice test to get a feel for the testing center, work some more on prep and find out exactly where the test site is (kind of a dry run).
Overall, I dont think it's worth doing. At the end, you get a sheet nothing but a percent correct. It means nothing. If you want to see the questions again with the correct answers you can get that from NBME on the free test that comes with the orientation CD. To my understading, Kaplan has a document with the answers to the practice test. I dont have it but I know it's around.
I would suggest just doing the questions for free at home and using the NBME self assesment tests instead. The self assesment is nice because you get a breakdown by topic as well as a score that actually correlates to the Step 1 scoring.
Just my $0.02.