I used him once, for April '08 and I thought it was worth the money once. I got a lot from his explanations of _why_ he made particular suggestions, and that was the most valuable info. Now that I know the "why" of things, I can make better decisions before I go and while I'm there, whether I plan ahead or not. He was only wrong one day out of ten.
I'd say it's definitely worth it if you've never used TGM before.
Next time I won't bother, but I'm tempted to try the unofficial guide because it's cheaper, but I have no idea if it's good or not.
I also enjoy the WDW today podcast, it's just fun to listen to over Saturday breakfast to dream of the next Orlando vacation.