Be ready -- you're probably going to get a lot of contradictory opinions on this topic! I think a big part about deciding whether or not to go is to manage your expectations. Specifically, the crowds can get quite large for the party, so don't expect to have the parks all to yourself where you can just walk onto rides: depending on the night (and I would advise, if you go, go on a weekday, as locals tend to go on the weekends) it can be pretty packed.
That said, however, we took DD5 to her first
MVMCP 2 years ago, and we wouldn't consider going at that time of year and NOT going. We all (DH, Grandmother, DD, and me) absolutely loved it! Not so much for the rides, but for the absolutely magical, holiday atmosphere. Even more so, IMO, than during the day. You've got the fantastic parade that you won't see any other time (i.e. during the day), ditto the incredible fireworks. The shows are loads of fun, the snow on Main Street is beautiful, and everything is just so festive. I think the key to enjoying yourself during one of these parties is to decide beforehand if you're going to focus on rides or shows -- it's very difficult to do both. We hit a few rides in Fantasyland right after they opened it up and before it was crowded, and then spent the rest of the night mostly doing the shows, meeting many different characters, and just slowing down and enjoying the atmosphere. We had such a good time that we bought tickets for the last night of our trip, too, and focused more on the rides that time.
Hopefully, you'll get some feedback from others who have been a little more recently, but I think it's worth every penny!