It's been a while since I've read this book (this entire trilogy, actually), but I remember it being really cool, with lots of plot twists I did not see coming. It's definitely also still one of the creepiest book series I've read. So, if you want to read this or you often see it in your library and don't know if it's worth trying, it definitely is worth it. Just make sure you can handle creepy stuff because it did freak me out when I read it (though I'm pretty sure I was twelve or thirteen at the time, so now it might not be so bad). But still.
Age rating: 11+ just because it seriously creeped me out (and I typically handle creepy stuff really well), so younger kids might get nightmares. I know in the third book there was also one or two kisses but I don't remember them being bad. So yeah. 11+
But it really depends on the person.
Plot: Mysterious, world-wide rain causes mysterious plant to pop up and start taking over, eating people, and various other gross, creepy things.
The books get creepier and creepier as you go through the series, so if you read this book and get creeped out, you'll probably be even more so with the next one since it involves bugs, and then even more with the final book, as it centers around the cause of the world-wide rain, plants, and bugs.
So, if you've read my review (the only review for this book) and decided "Why not? Might as well try it," then prepare yourself for a serious case of the heebie-jeebies and jump into this series with both eyes covered. You'll be glad you prepared for the weirdness.