This gem of an adventure has a real "old school" feel (in terms of ambiance, it really reminds me of the adventure at the back of the old Palladium Fantasy 1e RPG), but boasts some of the best production values I've ever seen in an adventure module. The artwork is especially noteworthy - it's absolutely gorgeous (quite possibly the best I've ever seen in an RPG book).
Despite some minor editing gaffes, the adventure is made of pure win. The book is laid out to actually be used at the table (this necessitates repeating some stat blocks, but this also minimizes page flipping), the author makes expert use of skill challenges, and encounters are almost exclusively solos (designed to keep the adventure moving along at a fast pace by ensuring that combats don't drag on forever).
I don't play 4e anymore, but I am absolutely satisfied with my purchase of Trial of the Underkeep.
[4 of 5 Stars!]