In the last six years, we've been to WDW as a couple w/ no children, as a family, with cheerleaders, with family, and with a large group of friends. We are information junkies and have read Birnbaum, looked at Passporter, read the UG every year, and check out the good sites like this one. We used TGM last June for our trip with 35+ people - most of whom had never been to WDW, some of whom had never been on a trip that didn't involve visiting relatives. We used TGM and found new and good information. Even though it was the 3rd week of June and very busy, we never waited more than 20 minutes for any attraction when we used the TGM touring suggestions.
TGM is perfect for the disboarders because we ended up here because we are planner/researchers. Anyone who feel inclined to register or even just lurk the forums is interested enough in trying to prepare effectively for the trip - not everyone is like that. TGM gives good information about the "why's" so that you can walk into the park and follow a plan with success but you can also make informed on-the-spot decisions if there is a ride breakdown or something that captures your fancy.
The site can get overwhelming because there are links to so much good information and so you can not complete the thought or information. Make sure you go through all the new user information once from start to finish without clicking on the link. Then go through the Express Planner information the same way. TGM also has a special forum or club for each month - you should join the one for the month of your trip so that you get alerted to information specific to that time. Once you've gotten a good overview, go back and click on links or do searches to your heart's content and you'll do just fine.
We were very satisfied with the service but my understanding is that TGM is really good about refunding your money if you just aren't happy.