They are what you make of them
I went to my second Geowoodstck this year which had roughly 3000 attendies. They had many different things to do from group caching to seminars to vendor tables. I got to catch up with some old friends and met many new ones. Did a bunch of caching and saw a part of the country I had never been to. Next years Geowoodstock will be in Tennessee and will also be my honeymoon. However if crowds isn't your thing then a mega may not be for you. I enjoy going and I hope they continue for many years.