That's always a toughy. But really, when a company sends you a product to review, that's all you have to do. If they sent you the product to write an ad, then that's different. But be honest and constructive and it's useful for everyone.
Maybe point out or list pros & cons. Especially w/the cons, mention what your suggestion would be to improve it or what it needs that would make it better. And price is important. We expect different things if it's free, a gift, clearance, etc so mention that as well.
I found myself returning several products if it wasn't something I would buy or would want to get/give as a gift. My blog isn't totally a review blog, though, so I liked to spotlight products I loved vs review whatever came my way. I started getting very picky during the email process and if I had doubts at all, I wouldn't accept the offer.