The hook for this mission is fun and intriguing, with a fantastic set-up supplied by the Prince. I really encourage GMs to go nuts and roleplay him as zany as possible; your players will love it.
The sewer exploration is above average, thanks to good encounter design and some interesting flavour text sprinkled throughout. The conflict between the Almsman and Steel Wyverns is a nice subplot, and adds a little depth to what could've been a simple dungeon crawl.
My only real complaints are:
1) The final encounter is a bit easy (at least at tier 1-2). I always feel like the Almsman battle is the true "end fight", and that the Dretches have been tagged on to pad out the scenario.
2) Natural gas and spider swarms? That's just mean.