Initial thoughts:
Pros:
BUMPER CARZZZZZZZZZZZZZ!!!!
New spells are fun
No sponge of a boss that completely stops gameplay
More achievement awrds that are acually worth getting
Crates don't require keys to open
Cons:
Bumper cars are so unwieldy
No boss. Period.
Almost all of the achievements are just grindfests
DOOMSDAY, ARRRGH!!!!!
All of Merasmus's spells cause the thriller taunt. I wouldn't mind this as much if weren't for the fact that people just set-up sentries before hand and there's nothing you can do about it, escpecially it becomes melee only.
Overall, the new stuff is fun, but the current map choice just makes it unbearable thanks to all of the minis. ESPECIALLY when Merasmus casts one of his spells and you can't move for a good 10 seconds. Seriously, the map ruins it. I know the point of the event is to not take it seriously, but honestly, how can you have fun with *beep* chk-chk-chk-chk-chk-chk-chk-chk-chk-ad nauseum all the damn time? if they chose like 2 fort or something, THEN I would have a grand old time.
On an update related note, I wonder if Valve will follow through on having crates no longer needing keys, maybe just eventually outright converting to Dota 2's system, which would be awesome. It would mean no more worthless drops clogging my backpack.