Well, first off, this is written (I guess) for a younger group of readers. This make sense on a number of levels. First, most of the main characters are teenagers. Second, the book is written through the eyes of a teenager and Green does a great job of really portraying what a 16 year old girl really does think/say. It's also a book that puts a lot of effort into making the characters seem 'too cool' for many things. In my opinion, this starts to get really annoying after a while, though probably typical of a teenager in many ways.
So, probably the most important thing to know about this book is what it's about. Cancer...well, and love. Quick premise: teenage girl with terminal cancer gets through life and meets another boy with terminal cancer and they fall in love. I guess that's kind of important to know and probably a big reason why I did not enjoy this book as much I could have. See, it's really quite sad. First, it's cancer. Second, it's kids with cancer. Third, it's kids very sick, who might die, with cancer. So, for some people this probably might be a great theme though Green does a great job of not making it sadder than it has to be, but the story is still a bit of a downer.
Ok, let's get to the positives because it really has some great aspects. First, it's wildly entertaining. It's funny, and cute, and creative and a really nice read. Second, the characters, though 'too cool for everything and everything is ironic', are really nicely shaped and it's hard not to like them. Third, the story does have some great points to make about life and what's important/what's not. Fourth, because it's YA it's a very easy to read and I think I finished it in a week, so not a big commitment.
Overall, it's worth picking up if you like a good novel (even if YA). Like I said, easy read, fun to learn about the characters, just remember the book is focused around cancer and even if it's got some humorous parts, it can still be a tough one to get through.