As other's say it totally depends on the child and everyone has differing experiences. For me it was a very good experience. I was one of those kids who probably would have been too bored to pay enough attention otherwise (oh heck - I know I wouldn't have paid attention... I had notebooks full of drawings rather than notes from some of my mainstream classes...). I've also always been quick and regular classrooms/teachers aren't always equipped to handle a kid that finishes their work too fast.... (I did a lot of reading on my own, not terrible as I liked to read, but still I might have been better served by other school work?) I get the feeling I would have started hating school pretty early had I not been in the accelerated classes - ours were one day a week at a different school through middle school, some separate classes in middle school and all separate classes in high school.
As far as social issues go, I agree with Alpinerabbit.... leaving a smart kid in regular classes usually doesn't make them particularly popular or give them a rewarding social life. Being in accelerated classes certainly won't guarantee a kid friends and it can be competitive, but at least the kids don't get picked on for being the smart one or learn to not act as smart as they are do they don't get picked on.