Dapper Day is a ton of fun! This will be my 4th (and 5th, since it's two days now) Dapper Day and I'm super excited! It's an unofficial event, so there isn't anything different about the park itself (although last Fall, there was a separate ticketed Hors d'oeuvre event in The Seas, but it sold out pretty fast). But the atmosphere and camaraderie of joining in dressing up with so many others in the park is just a ton of fun!
They don't arrange an official group photo anymore because there are too many people for it, but usually smaller groups will organize on Facebook to coordinate a group ride on the Riverboat or photos of everyone wearing character-inspired outfits (Disneybounding), etc. There will be lots of photographers around, too!
If you're someone who enjoys dressing up even a little bit, I'd absolutely say go for it! It doesn't have to be anything elaborate - even just choosing a sundress instead of jeans or adding a nice hat to your outfit or whatever can be a lot of fun if you don't want to go all-out.