I've read all of them. The first book in the series was my least favorite, but you have to get through them in sequence to understand what's going on as the series progresses. If you don't want to commit to buying them, I am sure you can find copies at your local library. Or borrow a copy from someone - there's certainly someone you know who has a set of them.
The series is quite entertaining for a number of reasons:
1) there are characters who constantly remain throughout the books and you feel like you "get to know them",
2) there are plenty of antagonist-protagonist situations,
3) the imagery in descriptive writing is superb,
4) the imagination is excellent - you've never imagined or seen anything like it,
5) the vocabulary is not "dumbed down" - you won't feel like you're reading a kid's book at all
6) once you get hooked, you won't be able to put them down,
7) while there are instances in some of the books where the action is "slow", for the great majority of the time, they're a good read,
8) some parts will make you cry or cheer or be outraged or be afraid - in other words, the books are capable of eliciting emotional responses
All of those were true, but I think she sorta screwed it up in the last book. She seemed like she was in a hurry to kill off our favorite characters, and Hermione was always so emotional I started to hate her. I was glad when she got tortured in the last one, though. And then she thinks the romance is all 'suddenly we're in love' type of thing.
Just putting in my two cents.