I bought a cheapo 120ish dollar unit for a long trip (NY>MT>NY) and returned it as soon as I got back. I got two tickets on the trip and the thing was completely unreliable. Both time I was hit with always on radar, which should be the easiest to detect even for a crap unit, but it wouldn't alarm.
anyways I have doubts now about even the $$$ setups, so please report back on your experiences after you've used it a while.