I've been DMing since July of this year ('03), though earlier I was in a CoC game, which whetted by appetite for PbP gaming.
I'm now DMing about 7 games and participating in two. None have bombed yet, though one is going incredibly slowly (though that's partially because we're in a small environment).
It does take a bit more work than I had anticipated to keep them going, as delivering a good update for 7 games takes a lot of time. I'm willing to keep working at it if my PCs are, but I need a lot of encouragement from them. The more my PCs give me to work with, the better I think the game is. It's when I'm left hanging with people wondering what to do that I get frustrated.
I don't mind if PCs take the initiative, actually I prefer it. Sometimes I want the PCs help to keep the campaigns moving along if I'm busy. I surely don't mind them staging a harmless encounter with an innkeeper or blacksmith or beggar, if it keeps everyone engaged.
So, in conclusion, I consider a PbP game to be built by both the DM and the player. As a matter of fact it seems that in PbP gaming the players have a bit more responsibility in keeping the game going.