To Rate, or Not to Rate

I’m hoping you can help me out.  I’m trying to decide whether or not to implement a rating system for my book reviews.  Sometimes I think it would be useful and helpful, then other times I worry that my opinion of a book could be heavily influenced by my mood at the time I’m reading it.  Also, I’m afraid I’d give everything a 4.

So I have a few questions for my fellow book bloggers..

Do you rate your books?

If so, do you use a 1-5 scale, or a 1-10 scale, or something else?  

What are the pros/cons to whatever scale you use?

Do you have trouble deciding what rating to give a book?  Do you have MORE trouble depending on how you got the book (purchased, borrowed, recommended by a friend, sent by an author or publisher, blog tour, etc)?

Do you ever want to go back and change your rating, after you’ve had more time to think on it?

Do you find yourself frequently giving half points (or stars, or caterpillars, or whatever you give out!)?

Ok, now a question for the readers-

Do you find it helpful when a reviewer or blogger uses a rating system?  Is that important to you?