The CTY summer residential programs have been fantastic for my DD, and they've been nearly universally well-regarded here on the forums and elsewhere. My local Mensa chapter, on the other hand, was a group of pretentious twits (IMHO). I tried it twice and stopped. That may be chapter-specific.
I've sure every program has strengths and weaknesses, and supporters and detractors. Problem is - I'm sure very few, if any, have experienced more than a handful of them.
Keep in mind there are many, many programs that are topic-specific and/or geographically specialized. My DD loved CTY, but if I wanted to send her somewhere for math specifically, it would be Awesomemath or PROMYS or one of the specialized programs.
Also look locally. DD is in a fantastic AMRL math program run by the Carnegie-Mellon math grad students. It's very local/unknown - only about 30 students 6th-12th grade - and was under our noses for years before we heard about it. Do some hunting and you may find the same.
Here's a starting list -
Summer Programs I wouldn't bother with high IQ societies, mentioned above. I was a member of TNS (Triple Nine Society) and OATH (One in a Thousand) in the past - there are interesting discussion groups, but not really anything related to enrichment or development of non-adults.
I've never run into any restrictions other then money and time.
