I'd have to say it's all of the above. When I started, it was mainly due to the superior shave and irritation issues. Cost wasn't a big motivator when I started, but now it would be. Cartridges are ridiculously expensive and it seems to me that the only reasonable solutions are DE or Straight. DE blades are ~.10c-.12c per blade. (I haven't purchased blades for a few years now, but that's my recolection. Please correct me if I'm wrong there)

And of course the use of a straight is fun and quite a conversation piece.