The performance of GHaFaS is comparable to that of the largest radio telescope producing 21 cm maps in atomic hydrogen: the VLA. Over a field of 3.4 arcminutes in diameter, it produces a map of ionized hydrogen emission with a nominal velocity resolution of 5 km/s (11,000 mph) and an angular resolution limited by the "seeing" due to atmospheric turbulence of less than 1 arcsecond when used on the WHT. These values are similar to the best figures obtainable with the VLA. However, a good quality map can be obtained in half a night's observing with GHaFaS, which is considerably more time efficient than that on the VLA. Comparison with ALMA, the best system for observing molecular hydrogen, is not so easy. ALMA produces maps at considerably higher angular resolution, but over a much smaller field. Thus, at the moment GHaFaS is best suited to observing "local" galaxies, out of 100 Mpc (330,000,000 ly) distance, while ALMA is superior at intermediate and high redshift, in terms of angular and velocity resolutions.