I should begin by saying that I think the first Boss Monster game is absolutely fantastic. It served as a quick game if my friends and I were short on time, or as a game (or several) of decompression between longer games like Relic or Eldritch Horror. You do hit a point where it becomes easier to build your ideal dungeons, but when you're competing with the same 3 people almost every time you play the game, the dynamic changes frequently enough to keep it interesting.
That said, we couldn't wait to roll out Boss Monster 2 for the gaming weekend we take every six months. I will say off the bat that we tried combining it with the first Boss Monster and I do not personally like the chemistry between the two. To clarify: Boss Monster 2 is technically a standalone expansion. It can be played with the first game, but I don't think it should. The feel of the first versus the second is just too different.
Beginning with the Bosses, their Level Up abilities (which they gain when their controlling player builds a fifth room for the first time in the game) now feel much more like a Level Up ability in the traditional sense. 9 out of the 12 bosses have an ability that the player can use each turn for the remainder of the game. This really stands out from the first game, in which the bosses had a "one-off" Level Up ability. So from an atmosphere standpoint, I think that the Level Up abilities are more akin to what they sound like. However, I also think that they change the dynamic of the game. It's hard to feel good about playing with Kazanna, Genie of Games, and discarding your hand to draw three Spell cards when you level up if you're playing across from opponents who have Shellda, Kappa Princess (Level Up: For the rest of the game, at end of turn you may swap the placement of two Rooms in one dungeon) and Smoake, the Dwarfbane (Level Up: For the rest of the game, whenever you build a Monster Room, you may draw a Room card). It means that your Boss has a much greater impact on the direction you build and play, but it also means that some players will be hobbled by their considerably weaker Level Up abilities just because it's a one-off ability instead of an ongoing one. In Boss Monster 2, you're playing against other Bosses as well as other players, as opposed to the original Boss Monster, in which you were playing against other players and felt you had a comfortable chance based on your own skill as opposed to worrying about overcoming obstacles because you got a Boss that just doesn't measure up to the Bosses you're competing with.
The Spell cards are interesting to play with. Admittedly, after you play Boss Monster enough times, you sort of know what Spells to expect and I do not believe they factor in as much with experienced players who know what to prepare for. The new Spells are much more direct in their impact, if that makes any sense. They feel much more like a way to assault other players (or heroes) than ways to manipulate your own dungeon or your own hand. I think what I walk away with is that the Spells in this game are much more focused on either one of your opponents, or heroes who enter your dungeon. I do not necessarily have any beef with these Spells, persay. They just don't have the classic feel that the ones in the first game did. In the first game, you could bump up the damage one of your rooms inflicted with Giant Size or you could sacrifice a room with Cave In to kill a Hero. But with spells like Lightning Bolt! (Deal 3 damage to one Hero in your dungeon with the option to discard a Spell card to do damage to each Hero at the entrance to your dungeon) and Undead Minion (Choose one face-down Hero in your scorekeeping area and remove it form the game. Deal damage equal to that Hero's Health to one Hero in your dungeon) doing direct damage, there's something that feels, to me, less elegant about the Spell design for Boss Monster 2.
I very much enjoy the new Room cards in this game. I would argue that they are not better or worse than the Rooms in the first game, just very different. The new Rooms lend themselves to different dungeon dynamics, and this game is worth playing just for testing out different combinations. On a personal note, I think the Basic Room cards in this game are much more intriguing than the Advanced Room cards, but that is probably personal taste more than anything else.
To me, the element of Boss Monster 2 that Brotherwise Games nailed were the Heroes. It doesn't seem like a big deal, but the difference in playing the first Boss Monster with and without items was incredible. But also extremely difficult to plan for. Sometimes interesting, sometimes frustrating. But aside from the regular Heroes and Epic Heroes, there are also hybrid Heroes. Heroes like the Hitman (who opponents can pump up by discarding a room with a Thief treasure symbol) or the Druid (who gets additional health based on the number of Spell cards held in the hands of the player unfortunate enough to face this epic Hero). I think they bring some new flavor to the game that is much more interesting than the standard parade of Heroes from the first Boss Monster. Especially should you be unfortunate enough to lure The Brothers to your dungeon.
Overall, if you are a fan of Boss Monster, Boss Monster 2 is worth the purchase. It's a different game and a different dynamic, and one that I feel does not quite measure up to the first one. That said, I've played it quite a bit and continue to enjoy it. If you play the game enough, then it's worth having Boss Monster 2 for the change of pace. But if I have a choice between the first and the second, I am more likely to fall back on the comfortable feel of the first Boss Monster.