What are your experiences?
I've been running and playing in PbP games for the last five years or so.
My experience boils down to this...
Typically, half the the PbP players who ever join your games will vanish without a trace before the end of the adventure.
Typically, half your adventures will be completely abandoned for they are completed.
Typically, a short adventure (like "The Burning Plague", for example) will take between six and twelve months to complete, depending on the posting frequency of you and your players.
I'd love to play in or run PbPs again, but I far to cynical about them anymore to seriously consider it.