When I booked, they said there was a 1 day cancellation policy, so as long as you cancel a day ahead, you're ok.
It actually sounds really fun to me, but maybe I'm just easy to please. Every one of these things we've done with Disney, we've enjoyed. I like the idea of sitting and enjoying the lights with DD out of the way of the crowds, sipping my drink and eating a cupcake. Sounds fun! Worth $150? Eh, well, that's all relative. That's a dinner out for us, so not a huge thing and will probably be one of our cheaper evenings if it keeps us out of the stores. Just seems like a nice way to spend Christmas Eve. I probably wouldn't do it any other time of the year, but wanted something special for that night. But that's for every person to decide for themselves, obviously.