I'd only ever left a review for one book on Amazon before getting my Kindle, but I've started leaving quick reviews there for the freebies (seems only fair to leave a review if the book is being offered for free). Mind you one has been a negative review I only left because of the pretty questionable 5 star reviews it had been given.
I've been mirroring them on Goodreads as I like looking at the reviews there. I've read a bit about the carry on they've been having elsewhere, but never came across any of it on the site itself. It is a bit off-putting though as I use the site as a more reliable source of reviews than Amazon, but it makes you wonder if that is the case....