I have a 650b Schwinn Mirada. The rear rim was tweaked to the point of no return. No cheap replacements to be had. Managed to fit a set of 700c rims. Got a near new set of 650b Panaracers if anyone is interested.

Almost scored a set of 650b mountain bike rims. They would have fit nicely on my Nashbar frame. Disc brakes makes things easier.

The only drawback to 650b is the rarity. Cost of rims & tires is higher and selection is lower. I don't think they'd be much slower in the real world than cushy 700c tires.