I don't think it's necessarily a good or bad sign. Someone whose eyes wander might be bored, or they might be shy, or have ADHD, or autism, or they might find you so attractive they find it hard to maintain eye contact. I think you would probably have to look at a combination of other behaviors and clues, along with eye contact, to figure out if it's a good sign or not.

A guy I used to work with, told me about how the more eye contact he makes, the less he's actually paying attention to people. His take was, as long as he's locked on with eye contact, and nodding his head like he's agreeing, he can completely zone out. Personally, I tend to make less eye contact the more I am focusing on what someone is actually saying, but I tend to look down, rather than letting my eyes wander around.