I planned a trip for 12 in 2006 and I am currently planning our second trip (now we are 14) for this December...4 families total.
As far as the Grand Gatherings goes, you should definitely sign-up! We did all of them last year and here is my "mini" review...
The "Safari Celebration" is the best b/c it is fabulous! The safari was great, the theme was great, the food was great, they brought live snakes for the kids to pet, the kids got to play instruments with the musical performers and everyone had a blast! Dinner was served family style, so no one had to make trips to the buffet with the little ones.
The "Good Morning Gathering" was nice as well AND you get a FP for Fantasmic! We all had a great time there as well. Tony was a lot of fun! I think we are going to do an ADR at the Crystal Place this time instead though.
The "Magical Fireworks Voyage" was probably the kids favorite. The dessert was a bit lame and the boat ride was oozing cheese, but it was a good time! I am pretty sure we are going to do this one again too...only b/c the younger kids loved it!
The "International Dinner and IllumiNations Dessert Reception" was our least favorite. Our evening did not start off well b/c we could find "The Odyssey Restaurant It was not on the map and we were told by 2 different CM's that it had closed several years ago. We tried to explain that we had reservations for dinner there, but the CM's assured us we were mistaken.
We found a 3rd CM that told us it was converted to a meeting center and was now called "Odyssey Center". The Grand Gathering paperwork made no mention of a name change.
Anyway, we make our way over to the "Center".
We check-in, wait in the sun until it is time for us to be escorted to our table. The buffet was in the foyer area and had several different cuisines. Everything was mediocre, but not "bad". We had a table that sat everyone and that was what was important to us.
The program was not to our liking. It was a bit cheesy and the DJ was very lame..stupid jokes, bad music...that kind of thing.
I remember Goofy being there, but I honestly do not remember any other characters. The kids got to sing on stage and do a conga-line as well as a special mother/son and father/daughter dance. Honestly, the dancing was the best part of the evening.
After dinner, we all walked over to our special dessert reception and fireworks viewing. There was not seating. (I assumed there was seating... ) They served Mickey rice krispey treats and several other things...coffee, lemonade...no water...
We were RIGHT on the water and quite frankly too close for my comfort. I was sure that the firework boat was on fire and about to explode...it was VERY hot being that close to all of the fire they had going. The view was good, the globe looked great and the kids had a good time. We were not "jam" packed either.
(Just as a side note...my 4 year old nephew got his head stuck in-between the bars... and caused quite a bit of stress for all of us! My 2 brothers, my father and my DH just about busted their collective guts trying to free my DN's head. Mercifully, after much crying and sweating...his head was freed... I have read a few other threads where kids have done this and they had the kids go the rest of the way through and then just lifted them over. Just FYI in case you go and that happens! None of us thought of that...)
The Grand Gatherings are nice b/c they already have a table reserved for your group at a location with other big family groups. We had an issue with long waits for our ADR's at places like Chef Mickey's and the Concourse Steakhouse. Another bonus for these Grand Gathering events is that everyone gets a special pin. We did not trade pins or get in to that, but my kids really liked getting these as a special memento! For some of these events, you can trade in a TS meal from the dining plan.
We planned to have dinner or breakfast together each day. We were not necessarily in the same park on the same day, but everyone got together at least once each day.
Each family group looked at what events and things they wanted to do the most and then we had a "family" meeting to decide what we wanted to do. Then, once the schedule came out I put it all together in a spreadsheet and made our reservations.
We had a few "ground rules". No griping if you did not participate in the planning. No one is waiting on you to do "whatever" if you are late. We did not even try to move to rides and what not in a large group b/c that is just too many people!
Oh...and the Hoop-Dee-Do Review was wonderful too! We all got to sit together and enjoyed a great show
the same with Spirit of Aloha show and dinner at the Poly. We made a point of attending dinners were they would be able to seat all of us together.
I hope that helps! PM me if you want to know anything else! I would be happy to send my spreadsheet too if you would like to see it. Our first trip I made the mistake of "over" planning. We did breakfast and dinner together, but that it just too much time sitting and eating!
That is in addition to our sons and their families. All total there should be 22 people. 
So, what is the purpose of a Grand Gathering and why does WDW offer this? 
Are there perks for having a GG? MM ear hats? 
Special seating at the restaurant of our choice?
Has anyone done GG and is it worth it? 
BTW, only a couple of the people attending have never been to WDW before.
on our last trip Oct of 2006 and you get nothing special unless you count someone to help you plan your trip. You can do special things together like take a boat ride to see the fireworks after dinner but you are looking at a price for dinner and boat ride of $59.99 
for adults and not much less for kids. I think they should rethink the whole concept of Grand Gathering which I think they now call Magical Gatherings and give you something free. MM ears, tee shirts, fass pass card, mugs, popcorn 
