Garmin 500s are good. It's what many pros use. They are available bundled with a combined wheel/cadence sensor (GSC10) and heart rate straps. Additional GSC10 sensors are available on Amazon and elsewhere for ~$40 if you have multiple bikes and don't want to have to move the sensor between them.

GPS has position noise and the signals can be intermittently blocked by buildings, foliage, etc.. This is usually small and inconsequential for motor vehicles, but can affect the speed and distance more significantly for slower vehicles like bicycles. The wheel sensor improves precision over using purely GPS, which is why Garmin included the capability.

You can download a route to 500 and see the track displayed on the screen, though there is no map with roads and such on the device. You can follow that track and it will show your position on (or off) of it.