My product review policy is pretty straightforward. In my day to day life, if someone gave me a present I would never, ever speak ill of it. Even if I wasn't crazy on it I'd be grateful that they thought of me enough to give me something. The same thing goes for products that I've asked to review; if I don't like it, I won't review it. To be given something to then slate it would just be plain rude.
However, I feel this is a different story if I've gone out and bought something myself. I reckon if I've spent my money on something only for it to not live up to what I thought it would be, it's only natural that I'd want to talk about it.