How do you judge experimental?

It struck me while I was trying to get to sleep last night - when listening to experimental music or looking at experimental art, or even concept music and art, how do you judge it? If you can't judge it on musical skill because that 'wasn't the point', or on popularity, because that's 'not the point' either, then what do you judge it on? How do you tell a 'good' concept from a bad one?

The only way to judge, I suppose, would be on how well it fulfilled its desired intention, how well the concept was portrayed...but if it has done this so badly that you can't tell what the artist was going for at all, then they can hide behind a screen of superiority and self-righteousness, because you're clearly just missing the essence and meaning behind their masterpiece. So, surely it should be judged on how many people get the art or music. But surely that cannot be true, as if there's one thing I've learned in culture, it's never to judge quality on numbers: Bob The Builder got to the top of the charts.

An artist called Mary Riley started me thinking on this. How many times can artists paint canvases white and get away with it, ay? When will the art world stop nodding along and just say, without a hint of cynicism, 'That means absolutely nothing, and you well know it. Get a job'?