I used most of the newer lecture notes and there are definitely some changes between the 2006 and the 2007 version as they sent me new copies to replace the 2006 versions of the same books. How important the changes are is up for debate. In my opinion, I like having new editions because I feel that they are updated, errors corrected, and some things rewritten for clarity. 3 years is quite a bit of time with 3 following editions to have changes in them. Also, this is probably the most important test to determine what residency and where you go and skimping on 500.00 for confidence in accuracy and maybe getting a higher score due to possible improvements in the newer edition, when you are paying 20K+/year seems kind of odd.