Definitely fun for adults. DW and I went to it last year and had a blast. It's fun to trick or treat as an adult, the parade and fireworks are very well done, you can get on some rides with lower wait times, and just a fun atmosphere with smaller crowds than a normal day. There are also lots of rare characters to meet if that's your thing. We didn't watch the Hocus Pocus show since we're not fans of that movie, but people seemed to enjoy it.
We didn't dress up but got such a kick out of seeing all the costumes. A lot of families really go all out with their costumes, both kids and adults. This year will be our 3yo granddaughter's first Disney trip. There will be 5 of us doing the party, really looking forward to it.