The game's pretty great. It is in the style of the old dungeon crawlers from the '90s (like Heroquest or Space Crusade- although it is better balanced than the ones I played). The game moves really fast.
One player plays as the montsers, who come out of spawn points (gantlet style) while the heroes kill monsters, gain loot and try to survive long enough to take on the boss fight.
The core box comes with a bunch of heroes, but only one setup for the monsters (Kobolds and dragons). The expansions do worlds for the monsters (the one that just came out has a vampire boss and a bunch of undead).