At that moment, Kim Jong-un notched the diplomatic coup that had eluded him, his father and his grandfather for decades: In front of the world, the despot was treated as an equal by the president of the United States.
And that gets him...what, exactly?
On the flip side, Trump earned the right to claim that he went further than any other U.S. president in trying to work with the NK's. If Kim doesn't follow through, then Trump's got more political capital to push for much more "robust" U.S. actions against North Korea. The entire country -- the entire world -- will know that we exhausted every single other possible avenue to resolve the threat the NK's pose.
So ultimately, if we're talking about the propaganda value of this meeting, Trump actually came out ahead because the propaganda on his end may actually be useful.