Picked up a Bontrager speed only sensor the other week (http://bontrager.com/model/08895). Took it out yesterday for a ride. Seemed to work great. Wanted to mount to the rear wheel for trainer rides, but had to fork mount to get proper alignment (Titus Racer X). No problems with setting up the 500 to use it. Manually entered wheel diameter from measurement. So far the 500 has been a solid unit for me. Only have 4 rides on it, but it has done everything I need. Will do a more comprehensive test down the road to really check the speed sensor, but so far so good. For only $30 the Bontrager unit is pretty good deal. The bontrager sensor with a premium heart rate strap comes out less than the full 500 kit.