Firstly, I'd like to point out that my husband and I watch this WITHOUT our daughter! :o) I do think it's a show for sensible teenagers or young-at-heart adults, NOT for younger children. As with any show targeted at this vulnerable age group, I believe the parent/s should check out a few episodes first and make their own mind up with regard to their own child, but personally I would have no problem with a 15 year old watching this.
It's a brilliant show - well-written, clever, genuinely funny (without having to resort to gutter/toilet-humour), and yes, even has positive messages for teenager viewers. Yes, there's a fair bit of sex (especially in early episodes), and I was a little surprised at the way this is clearly presented as the norm. But here's something that (surprisingly) noone else has mentioned: the casual encounters are generally not portrayed as a positive thing - on the contrary, Jenna comes to regret having raced into things with Matty, and the teens in the show who ARE portrayed as more promiscuous are definitely not intended to be our favourite characters (I'm looking at you, Ricky Schwartz). There is definitely a lack of adult role models, although it is interesting to note that as the series progresses, some of these characters (especially Jenna's mother) become less one-dimensional. Similarly, the Mean-Girls-style bully at school is given a bit of back story that opens a little window of empathy for her in the mind of the viewer. We still don't condone Sadie's behaviour of course, but her motivations do become a little clearer to us.
On the whole, I think this is a great show, which should nevertheless be approached with a bit of caution by more conservative parents, or parents of younger teens.