Solo games work fine, but as others have already said they can get rather intense, so expect shorter sessions. Oh, and assign the playbook's Hx options to NPCs when there's only one PC! I've gotten a lot of milage out of that.
I've been PbPing more or less constantly since early autumn with this game, and it works great (assuming PbP works for you at all, that is!). One of the things that make this game run better than others, I think, is that the rules require so little handling time, so the fiction keeps moving forward at a very good pace.
We're playing on a google document, using a two-column table, with fiction talk to the left and rules talk to the right. Works like a charm, and is much more readable than playing in proper forum threads, in my opinion.