Oh man, this module is a lot of fun. It's a great sandbox with a lot of evocative scenes and encounters. The valley itself, while an interesting locale, is just a dressing upon where the real meat of the module takes place: underground.
Through exploring the churches or pursuing travel rumors, the players find themselves seeking a lost city buried deep under the labyrinth warren of the cursed demonworm. The demonworm tunnels was by far one of the best parts of the module; it's both a point-crawl with random encounters that are just as exciting for a GM as for the players- the best encounter was when my players encountered a hostile cult of demonworm worshippers and, through two successful scroll checks, convinced the cult that they were the heralds of the worm which lead to some amazing role-playing.
I liked that this module had a bit more of a gonzo element (a la Hieronymus Bosch) than pure grimdark- it made the encounters more memorable without breaking the core flavor of Mörk Borg. Under all the weirdness in the lost city is some extreme grimness: the end of the world, victims trapped in eternal drug-induced comas by wretched sick nurses, strange denizens that inflict terrible miseries upon anyone they catch. Cool stuff for sure.
The module is great ran as-is or there are great materials you could pull and insert into your other campaigns (again, those demonworm tunnels are great and that's worth the price of the module alone). Areas like the horn tower are incredibly evocative and make for great inspiration. I'd also recommend running this for your grizzled, veteran Mörk Borg players to catch them off guard (it pulled a lot of surprises on both my experienced and new players who played through it). I very much recommend this module.