I've been in a C&C PbP game for a little over a year now. The pace is definitely much slower than anything else I've ever done before. The upside to PbP is that people can take the time to be more thoughtful with responses, and so the game reads more like an action novel than a transcription from a traditional game. Our GM is pretty awesome, so that really helps. I think the other key has been that the players who originally signed up check in at least once per day, which keeps things moving pretty quickly.
I still much prefer F2F or VTT games, but PbP does offer a unique experience that is more narrative in nature with the right group.
I'm actually starting a PbP game right now that uses my "Blood, Sweat & Steel" rules to play a Sword & Sorcery game. Combat in my game is much more streamlined than D20 combat, so I hope that will help get through the crunch as easily as possible in a PbP format. I've never run a PbP game before, so that alone should keep it interesting for me. I'm also really stoked that I found a bunch of players interested in playing my game. 8)