I have been once during my thru in 2010, It was a great time, basically a three day party. I have lots of memories from it but at the end of three days I was ready to get out of there. While it is a big giant party there are other things to do/ or could happen.
You could met up and reconnect with someone you hiked with ten years ago, a priceless thing if you ask me.
The parade is a blast, I mean really how often do you get to be in a parade! We had a water ballon fight going on at the same time.
Also I recommend going on the creeper trail bike ride, You basically don't have to pedal and get to coast down for an hour or so.
Also there was a big drum circle and fire ring dance the last night, while alot of people that par take in partying par take in the dancing, I was dead sober the last night and still had a blast watching others and being a part of it.
If you are into backpacking gear, you get to meet cottage manufactures in person and check out there gear, talk to them ask them questions.
At the same time a Long trail thru hike could be a great time, you could make new friends doing that. Either way I think it is a win win.
As an aside I was with my GF's brother hiking Katahdin this summer, he was celebrating his 20th Anniversary of Hiking the AT. Half way up the mountain we ran into a guy he had hiked with for a few weeks on his thru hike 20 years ago, he was hiking Katahdin as well celebrating his 20th anniversary. I know it was priceless for the both of them. You just never know were you will be, maybe some one you know will be hiking the long trail or going to trail days or both or neither!