After making the sandals, I just walked around in them for a few days for two reasons. (I got some of "those looks" when I went out in public.) One was to master tying them onto my feet. It's kind of like the Goldilocks story. One way is too tight. One way is too loose. You want to get them just right. The second reason was to get them comfortable. That knot in the end of the lace may be under the sole, but it can apply some painful pressure to the base of your big toe. You have to hammer that baby as flat as you can.
It takes a bit of doing, but I found that if you tie the knot just right and hammer it flat in the right direction, the knot will be in just the right place forward of that toe knuckle and you're home free.
The asphalt was a piece of cake on my first run with the sandals so I went out on some trails. I was concerned about debris getting stuck between my foot and the sandal. Having the callouses from running barefoot helped a lot so the sand and dirt didn't bother me at all. The occasional pebble that gets inside is easily removed with a slick kick out motion. Overall, it went very well. Dirt, sand, grass, pine needle covered pine cones, and packed gravel were a breeze.
The only difficulty was running over loose rocks of various sizes or landing on the lone pebble on a hard surface. The 4mm soles don't protect me from that which means the barefoot running rules apply. Watch where you're stepping. The sandals are light so it's just like running barefoot but more comfortable. You're feet still get the feedback from the surface you're running on.
With the change of season coming I'll find out how they and my bare feet do in the cold weather.