This was my first Jerry Spinelli read and for the most part I enjoyed it. I'd give it 3.5 stars if that was possible. :)
I can see why kids like his writing style, which is simple and straightforward, yet interesting. The issues he tackles in this book -- identity, conformity/nonconformity, and peer pressure -- are all relevant for middle school-aged readers, which is who this book is best suited for. Also, I think he does a good job of sending the "right" message without being preachy.
At the same time, though, I wasn't totally invested in the characters. I kind of felt like Stargirl was a bit too "out there" and sometimes though it was difficult to garner sympathy for a character that was so totally clueless. Well intentioned, yes, but at times too much of a fantasy. I also felt somewhat torn in my feelings towards the narrator. On the one hand, I thought he was very "normal" and his reactions were what I think mine would have been. At certain times, though, he seemed too spineless and and emotionally detached. I suppose, though, he probably was representative of most 16-year-old boys.
All in all, I liked the book a lot but felt as though I would've liked something that was a bit more grounded and less abstract. I found myself comparing this to the much darker The Chocolate War and the themes are quite similar but I found that book to be much more realistic. For more mature readers, I would recommend that instead.