I knew going into the movie that it was drastically different from the book without knowing anything specific, but holy cow, I didn't expect it to be practically unrecognizable. It's different from the very beginning, introducing a named resistance that doesn't even exist in the second book (I haven't read the third yet, so if it makes an appearance later on, I don't know about it) as well as concocting an entirely different incentive to get away from where they are. In the book, they're told they've been infected with the flare and they have to get to the safe haven to get the cure. In the movie, they discover that the other kids who have been rescued (also not in the book) are being hooked up to machines that put them into this half-life state in order to suck stuff out of their brains. Now, it's entirely possible that they gleaned stuff from the third book to put into the second, but I have no idea. Just don't go in expecting to see the book like it stands.
It does okay on the YA action front, but characterization is still nonexistent, and the logic is threadbare at best. Acting is mostly superficial and unmemorable, too (though I did like seeing Lili Taylor and Barry Pepper pop up).
Would I recommend it? Not really unless you read the book and have to see the differences like I did. Otherwise, wait until it's streaming/on dvd if you're interested.