It was about time I read this series ! This is a middle-grade fantasy series, I don't think anybody need an introduction about these books.
Well, if you have already seen the movie(s) and not read the book yet, do not judge the book on the movies because these are pretty bad (same as Eragon!). The books are really great, they are quick and fun read.
As far as the plot is concerned, even if you can nearly always predict what is going to happen, the author may surprise you with some plot twists. Percy and his friends are each year going on an adventure, and you know it before starting the book, you know they are going to get into tricky situations. Still, this is always really funny, even the tragic situations are not that tragic. I mean, bad things happen like always, but you do not feel the tears coming, the books are pretty light in that sense.
The characters are really likable (most of them at least), Percy is not that smart at the beginning of the series but he is still young, and you can watch him evolve which is pretty cool. Annabeth is really smart but she can be a bit annoying (not Hermione-like though, to go on with the Harry Potter comparison that everyone seem to make). I am not going to talk about the other characters because you will meet some new characters through the books that I should not reveal the name of to avoid spoilers.
One of the things I like the most, was the titles of the chapters. They were always really funny and you're wondering how the characters will get in this situations, this always seems really weird and then it always happens as predicted (well, I do not know if you can follow me on this one, but if you've read the books I guess you understand what I am trying to say.)
I recommend this book to everyone who loves middle-grade books. I read the entire series in more or less a week (during finals, so I guess that if you're on holiday, you can do it even faster!). This is a 5-book-long series and I did not even grew tired of it. Most of the time when I marathon a long series, I need a break. But it did not happen here, I just could not stop reading, these books are pretty addictive!