A quick run-down on my biases: I'm definitely a Lovecraft fan and Arkham Horror is one of my favorite games. I'm not such an Arkham Horror fanboy that I don't see some of flaws of some of the game's mechanics, but I am enough of a fanboy to still let the theme and mood of the game outweigh all of that and I won't turn down a chance to play this game.
I am assuming that you are familiar with Arkham Horror at this point, so I'll just do an overview of what the expansion brings to the game. The King in Yellow is a small expansion based around an insane play of the same name that has come to the city of Arkham. There are two variants of play. Either you can use the expansion as a touring production and set up to for the cards from the expansion set to be used first, focusing the theme of the game around the horrors and insanity that comes with the play, or you can have the play be a permanent fixture in Arkham and mix all of the new cards and encounters in with all of the existing sets fully, letting them come up at more random intervals.
Arkham Horror is already steeped in theme and this expansion only adds to it. I am a big fan of all of the references to the King in Yellow play (stemming from my experiences from playing the Cthulhu RPG) and I always enjoy the opportunity to read out "Have you seen the Yellow Sign?"
To be technical, however, adding the King in Yellow isn't a pure Lovecraft theme addition. The original references to the King in Yellow were written by Robert Chambers. Lovecraft took some of his references and alluded to them in his stories, but they were not his originally, merely incorporated into his mythos. However, it has become a well accepted portion of the Mythos universe and it fits the theme and mood of the game and setting perfectly.
The additions for the expansion include: 4 new encounter cards are included for each neighborhood in Arkham (for a total of 36 new location cards), 24 new Gate cards, 27 Mythos cards, 19 item cards, 22 unique item cards, and 15 spell cards. All of these add to the existing decks to further the theme of the performance of the King in Yellow in the city. The expansion also adds a few new things to the set:
The Herald Sheet (The King in Yellow): The Herald is a powerful being and force that is preparing the path for the arrival of the Ancient One in the game (and it is most fitting if you have the Ancient One you are facing be Hastur). The Herald adds to the difficulty and challenge of the game (and, in my opinion, fun of the game). With the Herald comes 10 yellow sign tokens that are used whenever the terror level increases. The King in Yellow Herald forces the investigator to make a choice whenever the terror level rises. Either they can place a one of the yellow sign tokens onto the doom track of the Ancient One, bringing his arrival all that much closer, or they can choose to place the token on the just vacated spot on the terror track as it rises, which then forces the group to draw another new facet of the expansion, the Blight Card.
Blight Cards represent prominent people in Arkham who are slowly being driven insane by the performances of the play. Each Blight card that comes into play has two effects: First, each card describes an event or occurence that changes gameplay never in the favor of the investigators, such as item prices rising throughout the city, or blessings be much more difficult to maintain during upkeep. In addition to these dread occurences, the character listed on the Blight card is now insane. Now, whenever you see that character mentioned in an Encounter card at any location, you ignore the encounter and instead lose 1 Stamina or 1 Sanity.
Act Cards are also included in the expansion. There are 3 Act Cards included. The cards are turned over in order and the first two Act cards do nothing to hinder or aid the investigators, but they put you closer to drawing the third Act. If the Third Act is reached, a huge charity performance of the King in Yellow takes place in the Arkham, driving everyone mad and destroying the city. These Acts are progressed by drawing one of the 6 "The Next Act Begins" cards in the Mythos deck. Each Act can by stalled instead of turning over the card, but the price is progressively steeper.
Playing the Game:
I have not tried playing the game as the Touring Performance variant, and so I cannot reveiw that. I eagerly shuffled all of the cards into my decks and played the Permanent Performance variant. I imagine that, other than the first time someone would play and was eager to see the new cards in action, this is the way it would be played most of the time.
The other thing that I need to mention is that this is the first expansion that I added to my base game. It makes a big enough difference to note that because of the 6 "The Next Act Begins" cards that are shuffled into the Mythos deck. The first time we tried the game with the expansion added, we had some tough choices to make as the Act cards kept threatening to advance if we did not stave them off. This was even as a Permanent Performance variant. The threat of the Acts advancing loomed in our games. In games since adding the other additions, those cards have rarely popped up because they are so diluted in the deck now. For instance:
Playing with just this expansion and the base set in a Permanent Performance means that there are 94 Mythos cards. With 6 of them being Act progression cards, that means there is roughly a 6.4% chance of advancing the Act on any Mythos draw.
By adding the Curse of the Dark Pharoh expansion into the set and including the 18 Mythos Cards in that set, you increase the card count to 112 Mythos cards, dropping your chances of drawing a card to about 5.35%
And if you also include the Dunwich Horror expansion (without Curse), you are adding another 36 Mythos cards. Which brings the Mythos card total up to 130, giving you a roughly 4.6% chance of advancing an Act.
And, if you include all of the current expansions (and when Kingsport comes out, it will dilute it further), you have reduced the threat of drawing a card to advance the Act to just a 4% chance. That is a 2.4% drop from my first games with the expansions and my anecdotal evidence has been that later games have not shown much of a threat of the game ending by means of the Acts advancing too quickly on us.
That said, even with the diluted Mythos deck, the expansion still comes into play with a full set of expansions in play. We've played games where the Terror Level has gone up like crazy and we've had 9 Blights out. That does not make Arkham a happy place to be.
I think that every time we've had the option, our group has drawn a Blight card rather than add a token to the doom track. The last thing you ever want to do is advance the Doom Track in the game. So, while the cards can be nasty, we've always seen it as a no-brainer in choosing them over adding to the Doom Track.
*More theme to a good thematic game (I like the gradual insanity of the townsfolk represented by the Blight cards)
*More location encounter cards, making them more variable
*The Herald idea is an excellent and fun mechanic (plus, Fantasy Flight has since included two more Heralds that are downloadable from their website)
*Blight cards are an interesting addition to the game
*A nice mix of new equipment cards and spells, most of which are relatively useful
*The threat of the game ending by the Acts advancing adds another component of challenge and threat to the game, making the investigator's plight all the more desperate
*The threat of the Acts advancing is by random draw and is diluted the more you expand to the game
*Location encounter cards are only for the base game, so there are no new Location encounter cards for the Dunwich Location (another universal flaw I see to the expansion mechanic of the game when new boards are introduced)
*The Other World enounter cards are pretty much as unthematic as the ones in the base set are, but that is a problem I see with the game overall, not just specifically with this expansion
Overall: The King in Yellow is a must have expansion for a great game. While not an overall earth-shattering expansion in how its new additions change the game, it does introduce the Herald which does have profound effect. The theme and style of the expansion fit nicely in the existing system. I would say that if you are a fan of Arkham Horror, adding this to your base game or existing expansions will only make your enjoyment of the game that much better (even if the math geek in me wanted to show how the percentage of some of the events will decrease with more expansions). However, if you are not a fan of Arkham Horror, this expansion will in no way change your mind about it. I easily give this expansion a 9/10.