I'm in my early 30s now but read the first two of the trilogy in my teens and loved them, I still own them both and they are more than dog eared as I've picked them up and re-read them many times over the years. Even as an adult reading a book aimed at teens they were still very readable and enjoyable, the style of writing was still suitable to the older market. So I can rule out my disappointment on finally reading the third book being because I was now a lot older than I was when I read the others.
I honestly think that the author should have left it as a two book series and not made it a trilogy. It felt as though she was over egging the pudding and trying to capitalise on what was best left as it was. The book mainly consists of how heartbroken Coll is in the aftermath of the breakup, I think the reader was left in no doubt she would be at the end of book 2. I didn't need to read an entire book about it. The end of the book leaves you with no real, further resolution to the one we were left with at the end of book 2 anyway. Yes, we find out Art really had felt as much as Coll but I think that too was pretty obvious and didn't need pointing out as a big 'ending' to the trilogy. Nothing in this book is a major revelation and she succeeded in telling us all of this 'between the lines' with the first two books.
I almost wish I hadn't read it as I felt it was so poor (story wise, her writing is as good as ever) it's tainted how I felt about the others which, even as a grown woman, married with kids, I still felt was more than a book but a useful tool for teenagers, to show them the way they feel about relationships/sex is normal.
It was drawing out a story that had, in my opinion, already ended.