I ran this in two sessions for a group of 4 PCs and everyone had a great time. The tone is unique, the monsters memorable, and the treasure evocative. The main issue I have with this adventure is the layout of the PDF making it difficult to use at the table.

Just sitting and reading the book before running, it read a bit campy to me with all the ridiculous rat hybrids. However the perspective is much different for the players at the table: it's hard to read a "spider rat" as silly when one has just dropped from the ceiling and is hugging your face. The panic and paranoia generated by these wild hybrids was much more effective than I expected, so I progressively leaned harder into the weird and dark elements as the adventure went on. This weirdness was all the more effective given the bone-standard fantasy plot hooks: missing boy went searching for a priceless artifact, rescue the boy and maybe find what he was looking for.

The treasure in the adventure is excellent. Even though I gave some of the items different stats and effects to fit into our game, the items each had enough personality that I could change the specifics and still keep the flavor. That's some good writing. A few of the items have built-in puns or jokes. Again these read as campy when read in a vacuum, but during the course of play they served as welcome comic relief punctuating the escalating horror of the adventure.

I very much appreciate the author's treatment of the eponymous Rat King and his familiar, Nibbles. For each, the author presents clear motivation and desires, giving the DM wide latittude to roleplay these as three-dimensional characters instead of dungeon obstacles to be steamrolled. Even the "insane," violent, kidnapping, mad scientist Rat King is somewhat of a sympathetic character if the PCs are able to get him to talk. Depending on how much of the cavern they've explored prior to that encounter and thus how much paranoia they are feeling, the PCs may even realize that keeping the Rat King alive may be key to preventing an even greater threat...

All of this plus high-quality maps in a PWYW package makes this a killer deal easily worth 5 stars. There were a few issues however. Most notably, the layout of the PDF was continually distracting to me DMing at the table. The text is very dense on these pages and the various rooms' descriptions are all visually similar in the "chambers and encounters" section, making it very hard to keep one's place while using the pdf at the table. My players experienced certain details in different places than they are keyed on the map simply because I accidentally read from the wrong text. I'm capable of changing things on the fly so it worked out, but I often found myself pausing to find the text I wanted to read. Given that this is an electronic PDF and extra pages thus cost nothing, I feel like the author missed an opportunity to add some space and art to break up the text and make it easier to use.

My other issue with the pdf is minor, more of a suggestion to the author. It's awesome that the maps are distributed as hi-res images along with the pdf, but that makes the relatively hi-res map in the pdf superfluous. Scrolling past the map in the pdf takes several seconds and on lower-powered devices such as tablets and chromebooks, again leading to undesirable delays when using this book at the table. My suggestion is to use the lowest acceptable resolution in the pdf when you're also distributing the maps separately.

Edit to add: I've since found PDF optimizer tools that can help with this last issue. I'd recommend them to others with the same issue.