Lords of Waterdeep: Scoundrels of Skullport (SoS from here on) is the first and only expansion to Lords of Waterdeep. Sporting two modules that can be used separately or together, along with pieces and rules for a sixth player, this expansion would seem to offer a little of everything to enhance the base game, but does it provide enough additional meat to be worth the effort?
Rules; or, How does this expansion mesh with the base game?
The sixth player rules add nothing to the game in terms of complexity. There is simply another player at the table, and little to nothing is changed in terms of setup. The easiest way to tackle the remainder of this subject is to address each of the two modules individually. The Undermountain expansion also adds no new rules, instead providing a small additional board that sits next to the main board, providing some extra spaces to send your agents. It adds quests, intrigue cards, buildings, and lords, but none of them require explanation. Undermountain is the “more of the same” type of expansion, generally with bigger investments providing bigger rewards. Skullport also adds quests, intrigue cards, buildings, lords, and a small board with spaces for agents. The major addition with this module is corruption. While adding little in the way of rules complexity, this over-arching theme of “get more stuff, but get corrupted and possibly lose a mess of points at game's end” permeates the Skullport module. You're either accumulating corruption or ridding yourself of it.
Gameplay; or, Is it actually fun to dig deep and/or corrupt your own innocence?
Taken as a whole, SoS adds some much-needed variety to the base game. While the sixth player can increase down time, having that option is always welcome (and on a personal note, the color it adds is gray, which is my color of choice in any game when available), and the difference in down time between five and six players is not significant. The Undermountain module and its “more of the same, but bigger” provides for exciting and substantial point swings, while the Skullport module asks “how badly do you want these extra resources?” Choosing how to answer that question provides more interesting decisions than the base game, presumably making the overall experience more interesting as well.
Conclusion; or, Why should you buy this expansion?
You should buy this expansion if you enjoy the base game but feel it lacks variety. You should buy this expansion if you often find yourself with 6 players.
You should not buy this expansion if you don't already thoroughly enjoy the base game, because it's still going to be Lords of Waterdeep. You should not buy this expansion if you have no need or or desire for a sixth player.