Fellow Author Etiquette: Reviewing Each Other's Books?

I have a few author friends and acquaintances who published books this past year, and while some have been great, others have been underwhelming. What is the best practice for fellow authors reviewing each other’s books?

If I’m trying to convey to my network that I stay up to date with the latest books in my genre, it would be best to review them, but if I don’t want to burn bridges, I'd better keep my mouth shut on some of them.

Has anyone else run into this issue, and what’s the best approach? Thanks guys!