It is worth it for me. We like to wear costumes, and since we are adults and don't like to go to bars at Halloween we don't really get to dress up otherwise.
I think Hallowishes are the best fireworks, and the parade is the best parade. I also like meeting characters that aren't out all the time. It is also a great time to go on rides with short waits and go trick or treating. Yeah, I could buy some bags of candy but it isn't as much fun as dressing up and going trick or treating at the Magic Kingdom then eating my candy while watching the parade.
Trick or treating, a character or 2 and seeing all the special show offerings can take up an entire party, so we like to have 2 parties to make sure we get to soak up the atmosphere.
If your not sure whether you'd like two or not, this might work for you.
Not all the tickets sell out in October ahead of time, so you might try to attend a party as early as possible in your trip, then if you really enjoyed it or didn't get to see everything you could grab tickets to a second night.
I guess that would depend on if you go in early or late October, most of the late October tickets do sell out way ahead of time.