They're light weight and comfortable to run in, and perhaps because of that they're fast. That being said, you need to spend some time acclimating yourself to this barefoot running style. You'll be working your calves and achilles more than you're probably used to because when you're running in these shoes you definitely don't heel strike, you want to land on the balls of your feet and let the achilles tendon and calf muscles act as the shock absorbers. I noticed a bit of tenderness on the bottoms of my feet as I started to push the miles up but that didn't last long, I've been running up to 12 miles in them now and don't have any problems. One other thing you have to pay more attention to where you place your feet when you run in these. The bits of glass, rocks and sticks you'd just step on in thick soled shoes are things you want step over/around in a thin soled shoe like this.