I still do my best to be 100% thorough, as Sapphire Storm and Sam said. The biggest part to making a negative review useful, I think, is explaining exactly why you didn't like it. Describe the features so that someone else who is different from you can still see if they would enjoy it. I think a really good example o fthis was Emma's review of the Calla
. It didn't work for her at all, but she explained which parts didn't work and why. She didn't spend an overly long time describing who would like it instead---that can get tedious, and usually most readers know what they like. So just, explain what it is, and if they'll like it, they'll be able to know.