@Kristee – I find commentaries valuable. If someone sees fit to brag on the author, then this motivates me to read the book.
Also, I like reading introductions. Sometimes, diving right into a book can be daunting, and it is nice to have some warning or direction before taking that step.
I don't feel that reading a review before reading the book shapes my perception of it at all. I have strong opinions, and they are not altered in any way by what someone else thinks.
It can be good to get background information on a story before reading it. Many introductions let you know useful information that actually will add to your enjoyment of the book.