UI is the second most important part of any application or service.The service or function is more important, but it's not important at all if users can't use it.
How to know whats good or bad? Thankfully, our old friend Cognitive Psychology provides us with the key. Just record a user using your UI (or use it yourself and make notes).
- Every time they try to do something that requires testing or a moments thought counts as negative.
- Every time they do something that doesn't require thought and was intended counts as a positive.
Every negative counts twice, that's how humans perceive negative impact.
The higher the score, the better it is.
Of course, this is only generally true, performing 200 actions to change a song on your MP3 player is not a good UI. Even if every step was intuitive.