Bought set to use on my 29er Dirty Kanza Race in Emporia, Kansas. Unfortunately for me I didn't make the lottery to race this year but have put these on my mountain bike anyway. The 1.5 inch width is much quicker on the Flint Hills Nature Trail and Prairie Spirit Trail. Nice ride and much quicker than the 2 inch tires I had on before. Smooth ride but I would not suggest using in mud.
My first ride was around a lake on a 5 mile gravel trail with some ruts, they rode so plush I thought I needed some more air. They didn't, they just have a supple feel and I had been used to riding 125 psi road tires. I weigh about 200lbs and I put 35 psi in them and they felt awesome!
My second ride was again on real fine gravel, almost sand but packed. I realized I was riding like I still had my vredestein road tires on. These WTB Nano Race 700C tires are so nimble and fast and don't require a lot of psi to get the speed. The trail was a bit muddy in spots however I didn't notice too much mud on me or the bike, they must shed mud really well, which is good cause I don't like carrying mud around with me down the trail. I did try them out on-road and they behaved very much like a road tire, I was expecting a slightly spooky feeling in the turns but there was none at all. Very pleased with them. They are 38mm but they class them in the 40 range, which is okay because I was needing to be able to fit the clearance issues I have on my frame. So that worked out great! Also, they are made out of some of the stickiest rubber I have ever felt. I am amazed with the quality of these tires and I'll get some more when and if they wear out. And lastly, these aren't your Farmer Johns from yesteryear, that's why they are called NANO!
I've tried a few different gravel tires and settled on these. The 40c gives a good ride wgood traction and durability and feels like it rolls faster than others. Im a larger rider and this tire gives a nice ride for me w47-50 psi.