Speaking personally, the things that give me the greatest satisfaction are those where I feel I've done a good job from a perspective of craft. Even if the code itself doesn't do very much or will never really be seen, I get the greatest contentment from feeling I designed and put it together well. If I worked on a video game that didn't get traction but that I loved, was beautifully built and designed, I think I'd be happier than say, an online poker game.

I'd be lying if I didn't care at all, obviously I'd rather the app be used. There's a special feeling of showing something to someone completely outside tech, saying "I built this" and having them "get" it. Hence why I think consumer apps are also more satisfying than a high frequency trading algo.

EDIT: minor grammar/missing words