If you have not looked at the disABILITIES FAQs thread, post 6 of that thread is about GACs. You can follow the link in my signature to get there or find it near the top of this board. Many people get a GAC and never use it, just kind of keeping it like insurance.
As was mentioned, many people have posted that they find using Fastpasses and a good touring plan (specifically TourGuide Mike) worked better for them than a GAC. Touring plans and GACs are much more predictable than GACs because GACs may not be handled the same on different attractions (in fact, the same GAC may be handled differently even on the same attraction on the same day because conditions are different).
There are alternate waiting areas for some attractions, but they may not be less crowded or less noisy.
This is an example of an alternate waiting area for a show, in this case Laugh Floor at MK. The alternate wait area is just a parallel area to the 'regular' waiting area, which is on the left side of the picture. By the time the show is ready to begin, the area on the left will be packed fairly well with people.
The smaller area on the right is for guests traveling with wheelchairs,
ECVs, mobility aids and we have also seen guests without those things who are using a GAC in that area.
If there are a lot of people with special needs, the area may be fairly full and we have seen people (mostly kids) hang on the chains and kind of stick into the right side area. If that is a problem, you can stay close to the wall. One of the advantages of using the area is that guests in that area are usually ushered into the show about one minute before other guests.
Turtle Talk at Epcot has an actual separate room. We have been in it when our group were the only guests and have been in when there were close to 30 people in the room (about 20x20 feet). With all the people, it was quite close and fairly noisy with all the talking.