Three years late to the scene, I finally picked up Mansions of Madness over the weekend, played it five times (as of this blog post, anyway), and went back to Rainy Day Games to get all of the delicious, terrible expansions. Partially it is because of the minis (I do love me some minis), but mostly it is that, to me, it better captures the feel and tone of a Lovecraftian story than Arkham Horror.
Do not get me wrong, I am a huge fan of Arkham Horror. I have owned it for years, played it quite a bit, and find it very fun, but it does not always hit the right spot. Yeah, there are iconic monsters and items, horror checks, spells that drive you insane, etc, but you always know exactly what you have to do, what the monsters and ancient ones are capable of, going through alternate dimensions is commonplace (and pretty harmless), I have experienced more than one occasion where a player runs up and down the streets butchering eldritch horrors, and have even punched out Cthulhu (as well as numerous other ancient ones).
Mansions of Madness does not roll like that. The game has a lot of unknown factors (for the investigators, anyway, muwahahaha) and is more story driven: the core game comes with five scenarios, each of which has varying degrees of customization that can change the direction of the story, location of items and clues, the results of certain events, and--most importantly--the goals of both the investigators and keeper.
Without multiple playthroughs of a scenario the investigators start out having no idea what it is they need to do in order to succeed; you have to explore the mansion, monastery, estate, or wherever you are in order to search rooms, solve puzzles, fight monsters, and discover clues that eventually reveal your goal, the entire time inhibited by the keeper. The keeper is an antagonistic player who represents a malevolent force working behind the scenes to fulfill an agenda, such as spawning a monster and having it escape, completing a dark ritual, or simply gobbling up the investigators. Unlike the investigators the keeper is also fully aware of what they need to do in order to succeed, and so can actively work to stop them while trying to complete its own task.
To do this the keeper uses a kind of currency called threat. The keeper gains threat at the start of each turn based on the number of players, which is then spent to activate keeper actions and mythos cards. Keeper actions are a set of cards that are determined by the scenario that let you do stuff like spawn monsters, move them, manipulate the investigators or take "samples" from them (always for a nefarious purpose), and draw more mythos and trauma cards. Along with the pool of threat tokens they are always visible, so players will have an idea of what you are capable of doing on a given turn. Well, kind of, because the keeper has...
Mythos cards, which unlike keeper actions are hidden from the players until used. These let the keeper cause investigators to run screaming from a location, automatically take a hit to sanity, refuse to go through doors for a turn, spawn monsters, and more; in a recent game a player went to gun down a shoggoth...until I played a card that caused him to drop his gun and his axe to rot away, leaving him completely weaponless. Often they cost some amount of threat, but some can be used for free, which adds to the uncertainty because even if you have no threat you might have a mythos card that will let you hammer them with a penalty. You can also add further injury to injury thanks to...
Trauma cards, which are extra penalties that you can slap on investigators when they take damage or lose sanity. These represent broken arms, loss of vision, growing paranoia or despair, and more. Some are fire and forget, causing a character to lose all of their skill points or to even commit suicide. Fortunately, kind of, the more potent ones require that an investigator be reduced to a set amount of health or sanity first (the suicide ones require that they have no sanity at all). Like mythos cards they add another unknown factor because you might lose more than a few points of health if you try to barge your way past a group of zombies.
Even the unpredictability of combat goes above and beyond a roll of the dice, and not just because of mythos cards or the fact that even the same type of monster can have different stats and special attacks. Each monster is associated with a specific combat deck, which you draw from until you find an appropriate weapon. The card then tells you what attribute or skill you have to roll against. Often you get expected results, like Dexterity or Marksmanship for guns, but sometimes it has you roll against Intellect because you "remember reading in a tome something about the monster's weak spot", and other times your weapon can get destroyed in the process.
So, yeah, the game can be quite difficult for the investigators to succeed; our investigator success ratio is 2 to 5. Personally given that it is trying to evoke a Lovecraftian story and all I expect that, but others might consider that a design flaw (including one of my players). Really though if you like Arkham Horror or horror games in general, then you will probably enjoy this game a lot: the scenarios are diverse, there is a lot of mystery, the monsters can be really brutal, you never know what to expect, things can start to seem hopeless pretty quickly, and the production value is really high.
I would say the only real drawback is that there are only so many ways you can play each scenario before the players will know what to do, but there are currently two big expansions and plenty of extra scenarios available, so that should keep you busy for quite some time.