The big box expansion does like in Arkham Horror and provides you with an additional side board, new Old Ones, investigators and more stuff. It is an expensive expansion, almost as much as the core game, and imo the value for money in terms of components does not really match the price tag. What you get in the box however is well made and a great expansion to Eldritch Horror - you pay for quality not quantity with this one if you need an excuse for forking over your money.
This expansion is extremely thematic and requires the attention of the players and travel to the sideboard especially if played using the Rise of the Elder Things as your Old One. The expansion provides a variety of thematic and storytelling elements that add to the gameplay experience, with things such as prologue cards and the 3 part expedition deck that gives you additional objectives on your trip through the frozen wastes of Antarctica.
Travel on the Antarctica sideboard is slow and additional items are available to improve your travels, such as dog sleds and guides. You also run the risk of suffering hypothermia but there are also new horrifying conditions such as infection and hunger.
We found that a lot of emphasis was put on the sideboard when you play with a setup including this piece from the expansion, and that is welcome. I remember one of my old Arkham Horror expansions, must have been Dunwich Horror, where you barely set your foot on that board.
Beside the Mountains of Madness themed board and components you also get additional cards for all other decks, such as items, debt, injury, spells, unique items and assets. All cities also get additional location cards which is welcome as that reduces the risk of repeating events on the same location.
The other Old One, Ithaqua does not require the sideboard and is perhaps viewed as a less exciting addition. He does however make the game quite challenging with his cold weather ability and the monsters he provides. In a sense it it also nice to have part of the expansion adding board space, while part of the expansion adding only gameplay elements depending on how much time you have to spend on a session.
There is really not much more to say other than Mountains of Madness build and improve upon an already good game, and if you are a fan of the Mountains of Madness story this is right up your alley. I only wish that the box expansions were priced more reasonably as I always struggle to justify a purchase of an expansion that pretty much matches the base game in price...
Eldritch Horror: Mountains of Madness 8/10 though expensive...