I considered 650b for my Raleigh Super Course, but @
nlerner pointed out there's no need, as I can easily fit fat 700c tires. Remember, larger diameter wheels smooth out bumps better, so there is a compromise when you go to a smaller size wheel. I think fat 700c wheels would be better than 650b. I haven't built the Super Course up yet, but you'll know when I've done it.
On the other hand, there are some very large dual pivot brakes available, and they work better than you might expect. Not enough reason to switch to 650b, but don't decide only because you worry about brakes.