This is a very evocative and fun adventure. It is self contained and can be set in pretty much any city big enough to have a sewer. There are also many hooks for you to build on, if you wish, e.g., one of the random NPCs become a sort of foil for our party. It's not over-written and it's not underwritten. Our group spent 5 sessions on it and there were still some unexplored areas. Worth every penny!

This is a top-tier module that would work as well as a location in a wider campaign as a one-or-two-session oneshot with minimal prep (which is how I ran it). It always does a bit extra to render a vivid picture of the setting, like how all of the city random encounters include descriptions that tie them into the established factions instead of just being stat blocks, but everything is kept succinct and easy to use at the table.

The undercity dungeon is a sort of abstract side-view that seems geared toward theater of the mind play (again, how I ran it), but the map helpfully notes room dimensions so anyone inclined to draw up a grid for encounters can do so.

The best thing I can say about it is that, a week after we finished The Well of Frogs, my players told me they wished it lasted longer. The people demand more Cassidum!