Actually clocking about 30-40km in the Kiprun KD900x, I find the ride kind of hard. I was staring at other shoe options (Adidas Boston 11/12, Asics Novablast 3/4, Saucony Endorphin Speed 3, New Balance FuelCell Rebel V4, Puma Deviate Nitro 2), settled with the Pumas. I tried it in two runs so far, one slow recovery run and one speed training run. On slower runs this feels like a regular, reasonably good trainer, but once you put more speed into it, it comes alive and really starts to fly.