First of all, they're untrustwrthy, as they're not an exact science at all. They're not objective, indeed couldn't be, so they're the subjective opinion of a person whose personality is hard to judge from the written word, and whose opinion can be influenced by anything from having had sex the night previously to having had to deal with annoying people, things which can seriously affect both reading experiences and analyisis.

Secondly, my own opinions can be like butter, melting before the opinions of perfect strangers. If I read a negative review of something I like, I start having second thoughts, thinking that maybe I didn't like the book as much as I thought I did.

I much prefer discussions of elements or themes prevailent in the work.