I did MHP* last year. It was the week before Halloween and we bought tickets for two parties a few days apart. Honestly, I wasn't a fan.
PROS
The atmosphere/ambiance was terrific. The lighting and the fog gave the park a really amazing look that's worth seeing in person, for sure.
The fireworks are spectacular. Possibly my favorite firework show at DL. Just simply breathtaking. I think buying a ticket is worth it just to go in and see the firework show, if nothing else!
If you're into the characters, there's some really unique ones out and about. While waiting for the fireworks to start, I got to see some pretty rare villains as they posed for photos near the train station. I know I saw Hades, which was awesome as he's one of my favorite villains, but I can't remember who else...
CONS
It's not so much that there's a lot of people in the park (though there are), but that the people who do come tend to stay in certain areas almost exclusively. For example, Tomorrowland was surprisingly packed during one of the parties I attended, so much so that you could barely get through.
New Orleans Square and Frontierland were also packed. It seemed like everyone in attendance decided to hang out in those three areas of the park only -- and, to a lesser extent, Main Street -- which left Fantasyland almost completely deserted. Kind of funny, as FL is usually pretty busy!
That might change depending on the day of the week you go, but was, at least, my experience.
If you're buying ticket(s) with the idea of having zero lines and being able to walk right on most, if not all, attractions, get that out of your head right now. I had that idea going into the party, and was really disappointed to find that wasn't the case.
Star Tours was pretty busy (as was all of Tomorrowland, as I mentioned above), so we decided not to bother waiting. POTC wasn't exceptionally crowded, but wasn't a walk-on, either; the line was about the same as normal. Haunted Mansion was insane.
The CMs were also really, really rude. I can't say I blame them for getting grouchy, as the masses of people they were trying to deal with weren't being very kind or receptive to them. Still, it's never fun to encounter one mean CM, never mind
several.
The trick-or-treating wasn't a big draw for me, though I tried it. Personally, I found it less riveting than "authentic," door-to-door trick-or-treating, due in large part to how limited the actual treats were. Each treat station had pretty much the same thing, and if you don't like the candy offered (I liked some of it, but very few; the overwhelming majority of it was stuff I never eat), it's really not worth your time.
Finally, MHP has a LOT of very pushy, loud, rude, annoying fans. That's not to say all the people that go to MHP are like this, because, of course, they aren't. Many were very polite (and a lot had some exceptional costumes that they made themselves!). But sadly, during both parties I attended, I noticed that MHP brings out a lot of creeps and jerks. There were people littering their empty candy wrappers all over the park (even with trash cans literally
two steps away), not minding their children, pushing people and young kids over, et cetera, et cetera. I don't know why this is, but it seems like MHP attracts some rowdy and mannerless folk; I've heard other party veterans say the same thing.
I'm not sure if this type of behavior would bother you, but I personally found it insufferable.
OVERALL...
Overall, it really depends on what you're interested in and what you hope to do at MHP. If you're wanting short lines for your favorite attractions, I personally wouldn't recommend it. If you're more into meeting characters (particularly the villains) and up for a great fireworks show, then you'd most likely enjoy it. But definitely don't expect super short lines or an empty park!
*MHP stands for "Mickey's Halloween Party."
