It's Disney, so it's going to be "child" oriented. Don't expect any scares or anything like that.
That being said, the party is a lot of fun. You get a ton of candy, you can wear costumes into the park(always awesome), the parade is the best Disney has in Florida, and the fireworks are pretty good too. On top of that, the park is significantly less crowded than normal operating hours. It's a lot of fun, and worth it, IMO.
If you're going to do it, just replace one of your normal days at the park with it. You can get into the MK at 4 with your tickets, even though the party doesn't start till 7. Use those 3 hours to hit the things that are closed during the party, and then enjoy the rest at your own leisure.