I have finished the first four by listening to the audiobooks, and I've really enjoyed them. The fifth hasn't been made into an audiobook yet, unfortunately. Anyone happen to have the e-book they could hook a brother up with? If you haven't read or listened to it, I would suggest it if you're looking for something in the light fantasy genre that easy to digest. The first book is similar to Harry Potter, but they get away from that in a very good way in books two and especially three.
I loved this series. Like you said, it starts off very similar to Harry Potter, but definitely goes in a much more violent and different direction than those books. You won't be disappointed in the final book, by the way.
I agree that it is definitely underappreciated, especially in the context of a young adult series that should probably be on par with some other more popular YA series.
Is there a lot of sex in it? Basically should I recommend this to my 11 year old niece who is obsessed w Harry Potter?
Like MORBO! said, I can't think of any outright sex in the series. There's definitely some more overtly sexual tones than HP, but no sex scenes. In general, I would say it's quite a bit darker than HP. There are certainly lighthearted and fun parts, but the lead characters don't hesitate to kill. Fight scenes are also way more descriptive and bloody than HP. Unless she's really super innocent, then I wouldn't say it's more than she has read before or can handle. Just wanted to give you a head's up.