This is a story that incorporates the Hollows world of Kim Harrison (aka the Rachel Morgan books) and the mystical world of Supernatural. Unlike other crossovers I've read, this one doesn't try and reconcile the two worlds into a single coherent one; the SN world and the Hollows world are clear and distinct.
The story is written from Rachel's PoV as she and Trent are (unwillingly) launched on a journey, struggling to find a way home. They meet the Winchesters along the way, and it's pure poetry.
If you love the way Supernatural can always be trusted to bring the action and the awesomesauce, and the way that Rachel Morgan has never, ever been a damsel in distress, and that Trent Kalamack is one cool customer who has a self-sacrificing streak a mile wide, then you will love this story.
On the more technical side, it's very well written and beautifully balanced. Also, long; it came in at 354 pages on my ereader and it's worth every second.