Anecdotally, for them at least, it didn’t pay out.
While I cannot say whether the Cavemen TV show showed real results (and while I would agree with the Fallon Planners that :30 of the cavemen is funny while 30 minutes would stretch my patience to anger-at-a-delayed-flight-on-a-Friday level), I would argue that bias gets in the way of any anecdotal response to an ad, especially if you are not in target and, further, that the fact that these ads became a TV show is ROI on a grand scale.
Though it isn’t targeted, the reach on the show is fantastic and the cavemen are clearly brand messengers, even when they have nothing to do with the brand:
I know no results on this, but I would be willing to bet the minds over at Fallon that Geico has seen a spike in new customer and, more importantly, an increase in base new customers since the buzz began. This is too big to be ignored, it is viral on the largest scale and an aspiration to ad makers the world over.