Disclaimer: If you're looking to read this because of X-23, then I wouldn't bother. She's in 3 issues, and doesn't say much in them. Just go straight to the Complete X-23 Volume 1 instead or something.
With that out of the way, NYX is a weird book. It doesn't feel very Marvel - it's gritty, and real, and very personal. There are no superheroes. No supervillains. Sure, there are mutants, but these aren't the X-Men. They're real kids, with real problems, like trying to find somewhere to live, or eat, or just not get shot by gang-bangers.
The first mini-series is written by Joe Quesada (yes, that one), and introduces Kiden Nixon, a teenager who discovers her time-stopping mutant powers and ends up running away from home. With the help of her ghost father (and that's never explained, so don't even ask), she assembles a new family - Cameron Palmer, her high school guidance counselor, X-23, a prostitute (again, don't ask), Tatiana, a mutant who transforms when exposed to blood, and Bobby Soul, who can possess people at the cost of his own memories.
These characters take the entirety of the seven issues to come together, but it's a story about found family and overcoming hardship, about how your normal might not be the same as everyone else's, and that's okay. Again, not your usual Marvel fare.
The second series is by Marjorie Liu, who would pilot X-23's adventures for the most part at Marvel, and brings these characters back, sans X-23 for no explained reason, as they're placed in the crosshairs of some very creepy folks. Unfortunately this story ultimately seems to go nowhere, as the series ends very abruptly, casting the characters out into the wind with no resolution to what the hell was going on. I'm a little disappointed, since there were a lot of questions I would have liked answered overall.
Helped in maintaining its gritty realism and detachment from the Marvel universe is the art by Josh Middleton, Robert Teranishi, Kalman Andrasofszky and Sara Pichelli, whose styles feel very surreal and different to the usual Marvel house style. The first series is especially bright and colourful, but it all still feels muted and weird, seen through an odd lens. It's hard to explain, but it really helps.
NYX is a strange book. Don't come for X-23, but come for the fact that it's unlike anything Marvel have published for a long time. It almost feels like an indie book with a Marvel stamp on it. Not sure if I'd recommend it to everyone, but it's definitely worth a look for an off-kilter X-book.