I have some experience playing in PbPs for 4e D&D, and I have to say it works very well. Now, that might not be the case for a more free-form experience (which is what it sounds like you guys are going for here), but I imagine that, depending on the system you choose, a PbP could work very well. One of the things I generally find very cool about PbP is that you get people who don't particularly like to RP when sitting at a table with other people, but being able to type out your speech and actions removes some of the embarrassment factor from saying things like "forsooth!" and whatnot.