I used to run a Terratrip 202 in my street car/rally setup vehicle. Odo probe (mech odo) or wheel probe (on the driveshaft) depending on the car. I never had any issues with it (nor competing with any number of Terratrips*). Your stock odo is usually accurate enough, but it is nice having a programmable odo for when you are on the other side of the border and working in funny units.
* the only issues were due to the car preparer (bad power connection, bad probe connection) or the road (probe and bracket knocked off by a rock).