Greetings from WI! We just came back from an 11 person Grand Gathering and found some pros and cons to the whole Grand Gathering experience. One of the pros was definitely the special events you can take part in. We did the Magical Fireworks Cruise and it was awesome!!!!! Well worth the money... everyone had a fabulous time. In addition, I would agree with the other poster that our group was always seated together at restaurants, which was nice.
Some of the troubles we had were that even though we were under the impression that our rooms would be near each other, they were not (different floors even!). I guess that is not a "for sure" with the Grand Gatherings. We complained heartily and were moved around a little bit (but still not really near one another... just same floor). That was a bummer. The other thing I will say is that while Disney touts these Grand Gatherings and the ease their planners will provide for your large group, their computer system really stinks and has trouble dealing with complicated travel plans. In our case, we had two families sharing a two bedroom villa, both families arrived on the same day, but departure dates were different and one of the families was splitting their stay between a two bedroom villa and a studio. This was more that the Disney computer system could handle and the one family (mine unfortunately!) had a HORRIBLE time trying to get set up for Magical Express and ressie for room got all screwed up several times (with people being left off, etc...). I'm talking hours on the phone with Disney dealing with managers/Guest Relations/anyone that would listen. It all worked out in the end, but was a real headache to set up. In my opinion, if Disney is going to market these Grand Gatherings, then they need to be prepared to handle complicated travel plans.
Despite this, I will set up a Grand Gathering in the future... really just to have access to the cool events they offer and for the dining part. I might make my travel plans a little more simplistic though, just to save myself the headache!