I'd like it better if your outs were stronger. I think his underbet could also mean a hand like two pair or a set against which you are drawing dead or nearly dead, so that you could be in big trouble out of position in a large pot.

With a flush draw, open-ended draw, or double-gut (my favorite), I think it's a much stronger play, because when you don't take it down immediately, you are still building a pot and concealing your hand for when you hit a monster. With 64, there are no monsters to hit.