CujobobI got an aluminum and copper Retrakt from the Kickstarter. They're the same top-notch quality as the Render K. The nock mechanism is smooth and silent, and I haven't had any issues with the body unscrewing in my pocket like the Render K does. It doesn't feel quite as special to use as the Render K, but it's much more convenient, IMO.
CujobobI have the copper version and love it, although it is heavy, which I like. Since it takes a Pilot G2 and Parker refill it's probably the most versatile of all the machined metal/Kickstarter pens out there. With some very minor modding I also got a Uniball Signo gel refill to fit in it.