We took part in two Grand Gatherings in Aug 07. We did the Safari and Illuminations (don't recall the longer, official names). I liked both of them. The safari dinner was, at the time, served as a family-style meal, but I understand now that it is a buffet. The Illuminations dinner was also good, but I liked the safari dinner better.
I booked my own rooms and asked for them to be in close proximity to each other. The rooms were fairly close, which was nice. I did not use the rest of the services offered through the Grand Gathering program, so I cannot speak to that.
We are going for Christmas this year and I will be setting up the GG Fireworks Voyage which sounds like fun.
Hope that helped...