This is a very light read, at least that's how I found it. Which was what I needed at the time so I'm not docking it for that. however, I can easily see how people call this problematic and also that this book has a lot of potential. Without going into spoilery detail, there are a lot of things that happened towards the end of this book that I found very interesting, and made me wish that I was reading a very deep sequel to this book and not the light-hearted, romantic prequel.
As for the Characters, I get my M doesn't have much in the way of personality, it's pretty clear that she couldn't develop like other people in her situation, but the love interest was kinda falt to. Olly was a nice, good person which is very nice to see, but other than the fact that they live beside each other and are hot, there didn't seem to be any reason for them to fall in love. From M's point of view, it makes sense, since he was paying attention to her but other than that (and maybe in the beginning when M is a mystery), I don't really see it. It just wasn't well developed. I could have easily gotten behind it if it was more developed, but they worked better as friends, or maybe unrequited love on M's side with the amount of development they got. (This could just be because I'm not used to Olly's actually being a decent human being in Contemporaries so maybe it does make a bit more sense but still it should have been developed more).
Development is my only critique though. Especially for that ending. You could see bits and pieces of it for a while, I always felt like something was a bit off so it didn't seem out of the blue, but I really feel like more should and could have been done with the ending, even though it wasn't the point of the story.
I did like the author's style though, and I'm not put off of The Sun is Also a Star. If my library gets it, I'd take it out. I also like the use of the illustrations and varied ways of medium. I found though that the mini book reviews were a bit meh and they seemed to be force-feeding M's opinions. They just felt a bit heavy-handed especially since there were some other heavy-handed elements in M's perspective.
In all this is a nice quick read. the writing style is easy to fall into and there are mixed mediums in the book that are very nice. The formatting of the sticky notes and the wonderful drawings and the messages and the emails were all very nice and immersive. It did, however, remind me of how little description this books sometimes had, but again I say that this is more of a light read which is what I needed at the time. Definitely get it out of the library if you're going through a reading slump, but it's not something I'd say would blow you away or that you should buy first.