Yeah, it'd be pretty terrible for play-by-post. But man, what a setting. I'm just always floored by the next juicy little tidbit I read.
Over on rpol.net I am a player in one, co-GM in another, and just recently started up one of my own... all PbP and all going very well. The guy I co-GM with and I have discovered that leaving City Generation to the GM (possibly with a feedback/suggestions thread at most) has done wonders for ensuring games don't stall out early.
Mind you i've watched no fewer than at least 8 GM's up and vanish over the past year... but that record works for pretty much any system. I like it VERY much, and it's far less intimidating than Mage.