Back in 1996, I went with friends to the second Mickey's Halloween Treat, which I believe cost $25 and was a lot of fun -- with trick-or-treat stations, shows, parades, and dressed-up attractions. Since they reactivated the Halloween parties and are marketing the heck out of Halloween Time, I read about how crowded the Disney parks are for the evening events, and with the price over $50 now, it just doesn't have any appeal.
Parks like Legoland have all-day activities with no separate hard-ticket admission, and there are lots of local activities around Los Angeles at places like the L.A. Zoo and the Aquarium of the Pacific included in the cost of admission.
It's brilliant marketing by Disney to convince the public in Florida and southern California that it's worth half the cost of a regular day ticket to go to a separate event for the evening, and it's obvious they are selling lots of tickets, but I'm just not convinced. If they go to a separate ticket for Christmas fireworks and the parade at Disneyland, we will sit those out as well.
Also, there are two great broadway style holiday shows, a Christmas saloon show, a Christmas train, a Christmas parade, a huge dancing Christmas tree, millions of lights, and wonderful food and desserts (their wassail cannot be beat).
SDC is great most any time of the year (except during the ultra crowded summer months) but at Christmas it just goes to another level entirely.
On the other hand, we also hit up our local park, Busch Gardens Williamsburg for HowloScream, which is not a separately ticketed event. There are rumors that it will soon become a hard ticket experience, but for now we are able to enjoy it several times a season on our platinum pass.
We also attend BGW's Christmas Town event, which may be the one thing the park does better than Disney. This is a hard ticket event, but with our season's passes we get 50% off, resulting in a price, last year, of $13 dollars. The park is open from noon till 10. Each section of the park is themed differently in decorations and lighting. There are several shows and several rides open. Typically they have a coaster open, but it tends to get cold enough by the middle of December that they can't keep it open for the whole event run. They also bring in outside vendors that sell goods in a market place. I think this event blows Disney World's various Christmas experiences out of the water and it is well worth the price of admission.
I had a terrible experience at Not So Scary last year- generally poor guest service and a feeling that the event was not worth the extra money. I won't be back. I have also been hoping for an east coast version of Holiday Mansion-- most people 'in the know' tell me that it will never happen though :( . I have not done Not So Merry, but I do love the Christmas Castle at MK, along with the other decorations at the park!
I always highly recommend SeaWorld's Spooktactular to families. It is included with park admission and is really great fun (for kids). Adults can trick or treat- but most activities are designed for kids. SeaWorld doesn't decorate the whole park for Halloween though, but the event seems to get a little bigger each year.
Last season I thought SeaWorld had the best Christmas event hands down! They park is beautifully decorated, shows are changed to Christmas themed, and new seasonal food is brought in. Just like Spooktactular it is included with park admission which also makes it the best value event in Orlando-- I hope they keep it up.
We attended Mickey's Christmas Party one year and although it was kind of fun it definitely was not worth the price of admission if you are already a Passholder because you get popped for parking for this regardless because it is a separately ticketed event.
There are plenty of holiday activities at Epcot and Hollywood Studios that come with your regular admission ticket for us.
I would definitely recommend Mickey's Christmas Party to FL residents that don't have passes. The event is capacity controlled so even though you aren't there the full day you can pretty much do every attraction you want during that time.