From the official Guild Wars 2 forum:
Head of Global Community
We want to give you a heads up about Phase 4 of the Halloween event. Phase 4 will start at 9:00 AM PDT/16:00 GMT on October 31, and will run until we publish a new build on November 1 (which will be sometime after Noon PDT/19:00 GMT).
Once Phase 4 starts, the Ascent to Madness dungeon will no longer be available – but you will still be able to enter the Mad King’s realm, including the Labyrinth, the Clocktower, Reaper’s Rumble and Lunatic Inquisition.
Also make sure that you will “Attend the Party” in Lion’s Arch once Phase 4 starts, and do not worry: It is a non-stop party, so swing by any time you want!
I've done Ascent to Madness on one character with guild members. Its considered a dungeon except it has no story mode. So when you go in it says Explore Mode. However, its smaller and easier to do than most dungeons. You get more of a reward the first time through. So once is enough for me.
I've done Labryth on several characters. My necromancer had the best results, upgraded half his armor. Mad King's Realm stuff is all level 80 but your characters is temporarily upgraded to level 80, except you do not gain access to elite skills if you haven't actually unlocked them. What skills you do have simply do more damage. Drops are mixed as usual, some will be usable by your character, some will not. Some will be right armor or weapon but of higher levels so you can't use them yet. There is a chance to get Mad King armor from chest but I haven't yet. Though I did get a weapon with a rune that grants bonus damage if its night time in the game (which is not necessarily connected to real time I've been on at 1 AM at its been bright as day or the afternoon and its dark).
I've done Reaper’s Rumble and Lunatic Inquisition once each. The last is kind of fun. You start out as a villager with no skills. You find things that will give you a skill or two. Your team gets points for villagers surviving. Once your killed you switch sides, becoming a ghostly servant of Mad King Thorn. Now you have skills which you use to "convert" villagers. To my surprise the villagers actually won more times than the lost when I played. I was always converted, one time by Thorn himself. I didn't die. I just got a message that I was worthy to join his side and I was no longer a villager. At one point I was credited with being the best trapper of villagers even though I only saw one villager actually step in one trap, because I was constantly on the move.
I tried the Clock Tower but as expected didn't get far. It a timed jumping puzzle. I have a hard time with non-timed ones. A friend told me he spent 18 hours repeating it to get to the top. The creator of this puzzle said he expected less than 5% of players to make it.
I'm jealous of a guild member who from his second Mad King Chest got the scythe skin, makes a staff look like a scythe. I've opened six Mad King Chests and over a dozen Black Lion Chests, no Halloween weapons.
I'm sucker for costumes. Can't figure out why, but I own them all in Guild Wars 1 and now all but the chef's outfit in Guild Wars 2, I'm waiting for it to go on sale. I haven't bought any of the real money armor however.
I also have 1 each of all the Halloween minis, the last one you have to forge in the Mystic Forge by sacrificing the other 3 plus 250 candy corns. So if you want to keep all 4 minis you have to buy the first 3 twice.
I haven't made any of the Halloween crafting items. I have the candy corn, 3 stacks, but not enough of the other stuff. The weapons recipes require items you only get from playing dungeons repeatedly and I haven't done all the dungeons in story mode yet.
Christmas (or Wintersday as GW1 calls it) is coming. Afraid to think of how many costumes or minis it might have.
Nothing to do with Halloween: I was disappointed that human NPCs in Divinity's Reach had no reaction to my charr showing up there other than the queen saying that a charr visiting her thrown room was a good sign for the peace treat.