In a nutshell: something unnatural is violently murdering people at the Mexican border -- both illegal immigrants and border-crossing officials. The Winchesters' investigation brings them into contact with colorful characters north and south of the border, and involves them with a hunter whose abilities rival their own.
If you're a Sam girl, will you like this book? Maybe, although there's not much in it to appeal to your inner fangirl. It's Soulless Sam who's inhabiting these pages, and as somewhat of a peripheral character.
If you're a Dean girl, will you like this book? Very possibly: almost the entire thing is told from Dean's POV. There's Dean whumpage galore, as well as a variety of other flavors of Dean -- the heroic, the snarking, the lover of Baby, the protector of Sammy. I'm pretty sure the author worships at the Altar of Dean (for which I blame her not at all, because ... so do I), and she's done quite a good job of portraying him in all his multi-faceted glory.
Are you wildly averse to having a kickass female hunter working alongside the boys -- someone Dean is ferociously attracted to, and would pursue if he weren't emotionally tied to Lisa? Think of her as a Buffy-like presence, strong and knowledgeable and extremely capable, but burdened with family issues. (Although visually, perhaps Eliza Dushku is a better fit than SMG.) If that sort of character makes you run screaming for the hills, then you'd best leave this book on the shelf.
If you're game to give this a try, though, I think you'll be more than satisfied. There are a few glitches that should have been beta-ed right out of there: the car having "breaks," and some punctuation and useage problems; the fact that this is a published, income-generating work makes that sort of thing doubly annoying. (I think I noticed about a dozen "oopses," far less than I found in "One Year Gone.") But the story itself is sound, and well-crafted. The author has a fine hand at tossing in colorful, interesting background characters, described in enough quirky detail that I could see them perfectly in my mind's eye. The story moves steadily along, with plenty of mystery and gore and "OMG, how do we get out of this???" moments for the boys. (Okay, DEAN.) I read the book over the course of two days and enjoyed it throughout, without ever feeling, "This person can't write worth a damn" or "Gee...a second draft might have been a good plan." One of her supporting characters, a teenage girl, didn't quite ring true for me, but that's a small quibble. I enjoyed the story enough that I'd honestly love to see it as an SPN Movie of the Week, which isn't something I can say about most of the novels.
It's fun. It's entertaining. It didn't inspire me to throw it against the wall -- and that's a really good thing, since I read it on my Kindle, and throwing a $130 piece of electronics against the wall isn't really something I want to do. If you've got some time to spare, and a yen to spend it with Dashing El Deano, go for it!