Certainly good enough for amateur robotics!
Great circle calculations won't work as well as the Haversine function, due to the inaccuracy of the single precision trigonometric functions.
However, due to the spherical approximation for the ellipsoidal Earth, all these approximations suffer from about 3 meters/km, or 0.3% error on average for distances greater than a few km. Considering that it is difficult to get GPS coordinates for any location with positional accuracy better than 2-3 m, it may not be worth the effort to take this much further.