Reading the other reviews, I think there must be two different movies being reviewed. This is a recent, but classically styled teen angst movie, not some kind of soft porn from the 60's as some of the 1 star reviews have stated. It's definitely accurate to give it a PG 13 rating because it does have mild adult content, but it does a whole lot more implying than anything too risque.
As to the film itself, the lead actors do a good job, and the support characters range from stereotyped (friends) to realistic (parents). The writing takes a harder look at what is more likely the common "first time" experience...ie. lots of fumbling, moments of insight, as well as moments of immaturity. It doesn't get bogged down too much in the backstory, but these aren't carbon copies of other teen comedy flicks, either. It has a bit of nostalgic flair reminding me of Cusack's Lloyd Dobler in "Say Anything" without the 80's cheesefest.
If you are looking for prat falls, horny jokes, or over the top behavior, this will bore you. If you are looking for a modern version of the classic romance that has a more realistic take on what most kids are doing...and that they aren't doing it because they are "bad" but because they are learning how to be adults, you will probably really enjoy this.
It was nice to see O'Brien hinting and more complex acting skills than the ones we see in most of his other work. Robertson is a little less polished, but she's still very good.