Well, I think it depends why you're going.
If it's to squeeze in a few more rides I would say no. Not every attraction is open and there could be lines. If you are going on the weekend or close to Halloween it can get quite crowded, more people that you might think they would sell tickets to.
If it's the idea of trick or treating the MK then I would say no. The Candy the give out is mostly lolipops and toosie roll type things, and the lines can be long and sort of impersonal. They have a few designated stations where you line up to get your treat - you do not go from storefront to storefront on Main Street saying "Trick or Treat!" like I had originally (and perhaps naively) invisioned.
However - I continue to go back year after year because of the different twist it gives to our visit to WDW. I love seeing everyone dressed up, especially the adults... some get so creative with their homemade costumes. It creates a special holiday atmosphere and people seem to be more freindly, chatting with others about their costumes and such.
There's a special themed parade that kicks off with the Headless Horseman riding down Main Street - all the Characters are in Halloween costumes - Main Street it self is decked out all spooky....a special firework show and the Ghost Stories told in Liberty Square are what keeps us going back to MNNSHP.