I think it's worth it if you're not going to the park that day, or if you're not using a multi-day pass. That way the money gets close to almost equalling out to a standard-hours day, and there are neat things to see.
The lines are long, but since they don't have FP merges, they move faster than normal SB lines.
The character M&Gs are a nightmare in some cases, not so much in others. The rare ones are tough, though. Disney has a good Santa, if your kids haven't seen Santa yet this year.
Just don't expect light crowds or a much different experience than you'd have otherwise. If you want to skip parades, ride lines can be shorter. Skellington, Sandy Claws and the Dwarfs are never short lines, so you have to decide how much of your ticket is worth waiting to meet unusual characters. I have a friend who says that Disney is all about the pictures you get, so if you feel the same way, some of those are M&Gs are worth a lot. Or not, depending on your take. But i don't think you can do it halfway -- you're either there for the rides or you'r there for the shows/parades/characters. It's a hard night to try and do both.
Oh yeah, here's something to add, though. If you want to see the MK at night, one of the parties might be the best way to do it this time of year, because with so many parties and so few non-party nights, I imagine those non-party nights get pretty nasty. So if you've had enough of daytime MK, it might be worth it to go to a party an see it in (maybe) less crowded conditions.
