It is interesting to compare TGM, the UG and web forums as sources of WDW touring info.
We have visited WDW at least annually for many years (how come we never hear about people adding up all the money they have spent in Orlando!??)
Herewith my humble opinions:
Web fourms - it really takes some time and effort to glean and assemble information from many, many posts. Frankly, it is sometimes difficult to separate fact from emotion.
UG - I have bought the UG, as well as the official one, nearly every year; I end up looking at the latter more often. To be honest, I need to say that the style of the UG really turns me off, while the official guide sometimes seems too much like a promotion piece.
TGM - out of curiosity, I signed up for TGM for a trip pre-Christmas. In a week or so of browsing, my reaction is pretty positive. I learned more things (both background info and tip types of suggestions) than from the other sources in some time. Also, it seems to be updated fairly frequently. The presentation style is OK. He explains the reasons for his suggestions. As mentioned here, it is a bit disjointed, although he is trying to cover a great deal of information in an inherently hyperlinked format, where is it hard to present a lot of material in a linear fashion.
I like it.