Just bought a 650b wheelset and put 2.1" (54 mm) tires on it to maximize the off-road capability and comfort of my bike. The places I ride gravel are hills with relatively steep grades, and my goals are to get a good cardio workout and some adventure - I really don't care about speed at all. My first ride on this wheelset is this weekend... I am expecting it to be fun.
When I do care about speed, such as on flats or road, I'll switch to a 700c wheelset with smaller tires.
Found a pretty interesting video which includes a couple of Hunt representatives giving their insights on their 650b wheels, and I got the impression listening to them that 650b was starting to fade away but started making a comeback (as of 2020). This is a long video (40 minutes) but I found it interesting.