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Only just heard of them
I've had a spin on a cannondale slate and loved it. Seems to get the smooth ride of a big tyre but accelerates faster than a 700x40c would.
Would definitely consider putting it on my bike if I change the wheels.
I think you'll find the touring and roughstuff crowd have been riding 1.5" and wider tyres for years, but didn't realise you need some marketing wonk to create and new sub-niche and give it a name to realise how good it was...
Thats not the point...
The idea is a 650b wheel with a 41c tyre is the same diameter as 700x23c so it makes it feel a lot sharper/racier than a massive 700 tyre but still comfortable. Best thing is you could have a frame that only takes up to a 28c and you could then fit a very big 650b tyre.
I'm a little with dovebiker on this - people like Jan Heine ( https://janheine.wordpress.com/?s=650b) have been running 650b large volume tyres for a long time, but i suspect that's stayed under the mass market radar as the focus has been on off-road.
Seems the new excitement over gravel bikes/racing has forced the market to look again at comfort and have re-discovered that large volume tyres are as fast as thin ones if the carcass is supple enough.
this might be of interest https://www.bikeman.com/bikeman-blogs/650blog/1771-650b-conversion-guidlines
It's all surprisingly interchangeable.
Grand Bois Hetre tyres are a bit of an institution for gravel roads in 650b; around 42mm / 1.7". They're made by Panaracer in Japan and mine came from Acycles in France.
Apparently, they've been around for many years; have had them on the 26" tandem for two summers, and find that they're 'sprightlier' than the original 26x2.0 road slicks that it came with. The tandem is run with the original 26" wheels in winter, slick on the back and a semi on the front. The outer diameter of the 42mm 650B is about the same as a large 26" tyre or small 700c.