I used TGM for the first time for our most recent trip. I must admit that, when I first subscribed and saw all the info "tailored to fit" me, I was a little underwhelmed... but that was probably as much my fault as it was the fault of all the hype I had read about his site here on the DIS.
Anyway, now that I've actually put his info to use, I'd give an overall grade of B. As far as a park-by-park breakdown (in relation to TGM's tips for each park), here 'tis:
Animal Kingdom--A (His tips were golden, right on the money. Of course this currently is the easiest park, of all the parks, to tour; and, of course, it will be changing slightly, but significantly, in about six or seven months, so I'm curious how easy to tour it will remain.)
Magic Kingdom--B+ (His advice here was almost as good as his AK advice.)
Epcot--B- (Here he kinda dropped the ball concerning the newest attraction--I can't go into detail--and his advice just generally didn't seem to hold as much water at this park, as compared to the others.)
The Studios--B+ (This is, admittedly, our least favorite park, but his advice did seem to be pretty much on the money... although it does seem that this is a pretty easy park to tour as well--if you can overlook the haphazard park layout, that is.)
To sum up: Would I use TGM again? Maybe, maybe not. At its core, his "secret advice" really boils down to one simple philosophy. And, like other touring plans, it can be awfully hard on the feet.
But, I can see why so many people swear by his site. So, at the every least, check it out.