I played in two different ones. Both were long-running, and still are running though I quit playing in them about a year ago.
One has been running since May 1999 (Ben Lomax's
Fight the Real Enemy!), and the other (Nick Pendrell's
Bohavia) is even longer (though there was a complete roster change at one point to start over at level 1--I was in the new group after lurking in the old one for a few years).
It's hard to keep a PBEM or PBP game running. Players sign up and don't post, players quit, DMs get bored, real life interferes, etc.
Part of it is just "magic"--the right group signs up (maybe not all at once, but eventually you get the right mix) and it all "clicks" and it's great. This usually means the DM has done a
ton of work (and continues to do a ton of work) and the players are committed--they read every post, pay attention to detail, and post a lot.
I think one thing that makes a difference is the posting frequency. If everyone wants to (and can) post at the same rate, the game has a much better chance of survivial. But if some players post twice a day and others only post once a week, it can cause problems. The "active" players will feel the game is going too slow as they wait for the weekly posters; the "inactive" players will feel the game is going too fast to keep up. And no one's happy.
The best way to join a PBEM is to lurk on it first, and see if you like it. Then join. Obviously, with a new game, you can't lurk first--but maybe you can read Story Hours or web logs of other PBEM games the DM has run.
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