Simple-ish question with a probably complex answer. If the frame can fit and I can afford a good, supple wider 700c tire (GB Cypres, Challenge Parigi-Roubaix or Eroica/Strada Bianca), I don't feel the need to look at 650b for that frame. But I've never ridden a frame designed around 650b, and only one of my frames fits a 42mm Hetre. Most of my 650b bikes are conversions from 700c with narrow tire clearances, that don't fit anything wider than 650x31-35mm tires. I'm a Clydesdale, so I can't get down to squishy-low floaty tire pressures, even on the wider tires, which probably negates some of the magic of really wide 650b rubber. I think this is very deep into YMMV territory.