My recommendation would be to do the pre-tests on Certiport's website, find out where your weaknesses are, work on those and then do the tests. The courses are good if you want to get it done quickly, but make sure you take the courses at a place where they also offer the testing so you can do the courses over the couple of days and then do the test.

I've passed the MOS tests for Word, Excel, PowerPoint and Project 2000. I didn't do them for 2003 because I didn't think the programs were different enough. I'm working on getting ready for the tests for 2007.