Just back from a 2200 mile loop through Arizona, Utah and Nevada. We both used cruise a lot on the trip. When it works, it's almost dead on. It seems to go +1 mph on up slopes, -1 mph on down slopes. That seems counter-intuitive. That said, we've both had problems where the CC kicks out for no apparent reason. When this happens, it will repeat a few times a minute. That is, hit resume, you're back up to speed for 15 - 20 seconds, then it kicks out again. Turn it off, then back on, and it's fine for hours.
As noted above, keeping it on around sweepers improves your riding. The only problem now is that, if I turn it off for the curve, I come out faster than before I went in!