I don't believe there is any way to objectively value a piece of art. I know some people may say "But what about pieces that do something completely new and then revolutionise their mediums?" but the thing is, when something like that comes along, later artists will adopt the revolutionary aspects and use them in their own work in ways which some people may enjoy much more. So while the piece the revolutionised the medium may be historically significant, you can't say it's better, indeed, taking the context into consideration like this is even rather subjective, to be truly objective you'd have to disregard everything but the art itself. The way I see it, every piece of art is loved by one person and hated by another, would either of them be wrong? I don't think so.
When I write a review, I always like to give whatever I'm reviewing a score out of ten, and I just want to be clear that the score is about how much I enjoyed it and not how close it is to some mythical idea of artistic perfection. My reviews are 100% subjective and I don't want to try to be otherwise because I don't believe it to be a worthwhile endeavour. Does that mean my reviews aren't useful at all? Well, I hope not. I like to think that if I write about much much I like (or don't like) something, there's at least a reasonably sized chance that somebody else might feel the same way.
(Don't miss my latest article for SmartDating UK on whether or not online dating is useful.)