I've been running a play by post GURPS game, for a little over three years.
I have 7 players and we average about 8 posts a day, according to the stats provided by our forum.
Some days, when everyone is on, it moves along pretty fast, some days not so much.
But as long as your players are committed to checking in at least once a day, and making a post if they need to, then the format can work.
It is too slow to really replace the tabletop experience with friends. We recently had a combat that took two weeks to play out, since we only got through about 1 round a night.
It is the methadone to RPG-smack, but it has some advantages. As the GM, you are never really put on the spot. If you need to draw a map, or come up with a name or description, or craft the perfect dialogue, or whatever, you can take 5 minutes to do so, and nobody is actively waiting for you.
I'd never want play by post to be the only format I play, but it's a nice thing to do during the week between in-person game sessions with my RL group.
As for the system. I don't know if D&D (any edition) is a good match for the format. It would take you many many months to gain a level, if the GM is awarding XP by the book. Even in my GURPS game, I give about 5 times as many Character Points per adventure than I do in my RL games, since it takes us six months, or real time, to get through an adventure, and I want the players to see their characters improve on a more reasonable realworld time scale.