I tested one for Kevin and measured 23 courses with it this summer. I really liked having the counter up where I could read it easily. Unfortunately my hybrid bike measures 80 cm from the axle to the handlebar. The cable had to be bent too severely to mount to the handlebar and I had to improvise a different mounting. The only issue that I had was the inherent lag in the cable and counter compared to my JR mounted on the axle and tied to a spoke. I had 12" of lag before the counter moved going from forward to revers or back. As long as you always take the lag out in the same direction, it is not an issue.
Looking at the mechanism, I don't see how you could get any slippage of counts without a bind in the counter. A bind that would most likely break the gears in the counter. Granted there are more working parts, and therefore more potential failure points.