"Good Airplane"

When you arrive to the airplane for your flight, it's common for the pilots getting off to say "Good Airplane", implying there isn't anything broken.

But what does this mean, and what good does it do to say it?

This saying isn't a guarantee that you won't find a problem the previous crew were unaware of, even if a post-flight inspection was carried out. The pre-flight is often a more detailed walk around than the post-flight and sometimes the crew will not do a post-flight if they see the arriving crew, since a pre-flight will shortly be done.
Therefore the crew telling you the airplane is good, may not have walked around the airplane after the flight.

Neither should this expression imply that there isn't anything you'll find broken, but rather that the crew were unaware of anything to mention and nothing was 'written up' in the maintenance log (AML).

On more than one occasion arriving crew's have found issues requiring maintenance after the previous crew have left, and said "Good Airplane !"

I think the best way to understand "Good Airplane" is to take out of it that they simply haven't written any issue up requiring maintenance, by "Good" the airplane may still have issues which will come up during pre-flight, and in no way should it affect the thoroughness of your pre-flight, were you can make your own determination of the airworthiness of the aircraft.