Many many moons ago, I tried to get a PBM going with the group of RM friends I had. It was when BBS's were still new and e-mail was this amazing new thing that only computers nerds and geeks knew about. I had figured it would be like a "Choose your own adventure" book. You read a page, make a choice at the bottom of the page, then turn to a completely different section of the book to see what the consequences of the choice were.
After graduation, all of my friends graduated college and went their separate ways. After not having anyone left to game with, I tried to find some way to manage. Of course it's not as fun or as hilarious as face to face, and not as immediate, but after 5 years of not being able to play ANY type of RM gaming session, it was welcome.
I just found the Real Roleplaying site that is in the banner ad on this forum and I checked it out. The problem I see is finding a group of players who will check in frequently enough to make the game play fun. Too much lag between posts and it loses the fun. With internet being so prolific in our lives and everyone getting email on their phones, laptops, tablets, in their car, the response times should be much faster than 30 years ago when you had to wait for mom to get off the phone so you could dial into the BBS on your 300bps modem and hope no one picked up the phone while you were online or call waiting kicked you off.
I miss playing/GMing RM so much, I am all for a PBM/PBP forum. When I am at work and there is a lull, I can send in my responses and actions or respond to the player posts.
It's not as fun as face to face, but it's better than going years not playing at all.