My daughter read them first last year. She is a voracious reader at 12. Then, coincidentally, my sister read them and loaned me her copies. I enjoyed the first and third books (second was a bit slow), but I couldn't believe she didn't warn me about some of the content that I would need to discuss with DD...it gets a little PG-13ish there (book 3, I think?). Don't get me wrong, we have had age-appropriate conversations about her body and sex since she was quite young, but I still like to be aware so that she can get the facts, along with our version of morality, rather than have her ideas about romance and love be molded only by Hollywood, etc.
We are excited about the next book...nothing like Harry Potter excited, but still looking forward to a summer read.