I'm a long-time TGM subscriber, but then I'm a planning sponge and buy every guide book, too. For that reason, it's difficult for me to answer the question as posed. It's worth it for me, that's the best I can say. I take from it what I want and ignore the rest.
What you need to understand is that Mike is a past Disney Tour Guide Cast Member and, for several years now, has been running his own tour guide business. He is in the parks every day with his guests and still has many insider contacts. I can't begin to imagine who could possibly have a better idea of crowd patterns.
His 'Least Crowded Parks' articles alone are worth the subscription cost, IMHO, particularly if you are visiting at a busy time. They go much further than the normal guide book observations of, for example, avoiding EMH days. They are day by day (for every day of the year) observations of the likely crowds based on his experience. In case you missed the significance the first time, it bears repeating - he is in the parks every day.
There are also some great strategies for the parades and shows, including where to get the best view.
The downsides? There is so much information on his site that it can seem overwhelming. I'm convinced many subscribers probably never find the will to wade through it. Just accept that you don't have to comprehend it all in one go. This isn't a personalised planner as such. It's a huge collection of articles from which you devise your own plan. There is also, now, a discussion forum - a small community - which definitely adds something.
Much of his crowd-busting plans revolve around getting to the parks early and taking mid-afternoon breaks. That isn't going to suit everyone. Bear in mind, too, that his touring plans are written for an American audience and Americans tend to vist for much shorter periods than we do, so his plans really pack in the headliner shows and attractions. This is a very simplified explanation of what he offers, though, and I'm certainly not suggesting that this is all there is to it. I've always maintained that there really is some unique information available to TGM subscribers.
What you can be sure of is that the information is accurate and up to date. Mike will comment on rumour, but he never incorporates anything into his planning articles until he has received confirmation from a reliable source. His contacts mean he is often ahead of the game on both the rumour front and the official line.
As an alternative, you might want to consider the Personalised Itinerary Planning Service offered by Simon and Susan Veness (at a discount to owners of a current 'Brit's Guide'). They also spend a good deal of time in Orlando and the parks so their observations are reliable. Theirs is a plan tailored specifically for your party and dates and, as such, is much more structured (good for some, not necessarily so for others, I guess). They take into account your preferences (you say you know what attractions you want to experience, for example) and the plan is, unsurprisingly, much more suited to the British visitor who tends to be in Orlando for two weeks and who wants to experience much more than just WDW. My one criticism (and it's a big one) is that you won't receive your PIP until around a week before your trip. This does, of course, guarantee that you have up to date information - crowds, weather, show times, etc. My preference would be to receive a draft plan when ordering and then a revised plan, taking into account all the current information, shortly prior to the trip.
I have picked up many tidbits from TGM which have warranted the subscription cost for me. For a visit during a particularly busy time, I don't think you'd regret the outlay.