Full disclosure: I'm The Angry DM, the one being blamed for inspiring this. But it was a throwaway remark on Twitter. Darklight Interactive took one random remark about rats drinking potions and turned it into an exciting and engaging delve with interesting, unique monsters and terrain features. This little side-trek is a great distraction to drop into a city (maybe to liven up a shopping trip between adventures, or perhaps the party comes seeking an alchemist's help as part of a larger quest, or perhaps this is even the way a party of neophyte adventurers come together). The layout, formatting, and maps are high quality and show the typical pride that Darklight Interactive puts into all of their products.
At $0.99, this is well worth the purchase. Seriously, DMs, drop it in your little folder of stuff to pull out when you don't know what to do next or when you have an hour or two to waste playing D&D, or keep it for that night when some of the players are running late and you don't want to let the party make too much headway in the main quest without them. It is also a great trek to rip apart and pillage for strange monsters with a magical twist (need something for Eberron) and gloppy, goopy toxic terrain effects that would be perfect for some crazy Artificer's ruined lair. I shouldn't have to tell you how valuable a side quest like this is.