Just a quick note for the handful of folks who had asked for an update when I had some more experience with them. After several thousand miles of pavement and fire road use, I'd say these are the best tires I've used. They are exceptionally comfortable, roll better than most tires I've tried -- including the typical "touring" 700c models -- have had no flats, and are showing essentially no wear. My only concern at this point is that they are the only tires of their type available, and if Panaracer stops production I'll be an unhappy camper. Hopefully they have a long shelf life; I've started hoarding them.
As for the whole 650b deal, I'm semi-convinced. I don't think there's any magic here; a 700c tire of the same construction would do, I'm sure, the same thing. The only problem would be massive toe overlap, so IMO 650b is simply the best way to get a fat tire without overlap. I wouldn't bother with a "standard width" tire in 650b unless the frame is very small, in which case 650c probably makes more sense.