I have two mostly-written posts waiting in the wings, but they have to take a back-burner to this.
I love the webcomic xkcd, and considering the topic of this blog, today's comic is very special to me. But then I read the hover text: "I favor approval voting or IRV chiefly because they mean we might get to bring back The Bull Moose party."
Okay; approval voting I'm in favor of. It's a minimalist form of score voting (score where the only allowed scores are 0 and 1), which is good, and since score voting is free of both candidate cloning and favorite betrayal, it could actually allow third parties, like The Bull Moose party, to have a real chance at victory.
But IRV is not good. IRV is strongly susceptible to favorite betrayal, and so is only a marginal improvement over plurality, in that all those third parties, like The Bull Moose party, will still be spoilers (yes, IRV has spoilers) before they become winners. (And that marginal improvement is before you start to account for the significant increase in complexity while counting the election, the potential cost of new machines, and non-intuitive results stemming from non-monotonicity issues.)