Going into this book, I was on the side that Snape is innocent. Coming out of the book, I'm still on that side (though wavering). Perhaps it was my original bias, but the case seemed much stronger on the innocence side of the book (even though certain items were repeated again and again). The guilty side seemed a bit light and even toward the end, the authors wrote just about Slytherin and not about Snape at all.
This book is really good for showing some very precise examples of how Snape could be evil or good. I went into the book thinking he was on the side of Harry and finished reading it on the same side. But it is really good at pointing out exactly how he could've been considered "evil."
I'll read anything by Orson Scott Card! This is a very interesting "unauthorized" view of both sides of the Snape debate. Of course, we have all the information now that the series is completed, but it is still an engaging exercise.