We went in 1999. We did not think it was worthwhile, but we seem to be in the minority. It was much more crowded than we had anticipated (think non-EE morning), and a lot of the crowd appeared to be unsupervised adolescents who did a lot of unwarranted pushing and shoving. The little kids we had in our party only got to ride a couple of rides (and that particular Dumbo wait was the longest we've EVER waited for Dumbo in 3 trips to WDW). Our kids were also very disappointed at the lack of characters and candy. They each got a few SweetTarts, a couple of Butterfinger BBs and a miniature Nestle's Crunch (bite size) bar. The youngest of the ones in the party (we had a couple of families together) were scared by the spooky music and the parade. The older ones really enjoyed seeing the Wicked Witch fly from the castle before the fireworks. Not all the rides were open. I'd say about 1/2 were open. Overall we thought it was not at all a decent value for the money and would rather have spent the $$ on tickets to Blizzard Beach or a good character meal or two. Again, though, we are in the minority and I completely respect that everyone has their own opinion!!!
No costumes are required, but nearly all the kids and at least half the adults dressed up. The key is to make sure your kids can move freely and that you can move freely enough to chase them (if you have kids)! Ha ha!
Have a great time if you do decide to go!!