A german magazine (perhaps it was Aufwind, I don't remember) had a comparison of inrunner to outrunner recently. Efficiency-wise it was rather inconclusive, and out of all the gear types the planetary gear was favoured because it does not offset the shaft and does not reverse the direction. One big reason why we'll still be seeing inrunners is gliders. Outrunners are impractical to fit in thin fuselages and the protruding wiring does not help either. Apart from practicality, the comparison clearly favoured outrunners as they are generally suitable for all but high-performance glider applications. And a heck of a lot cheaper.

I'm not experienced with gears but personally I find it fascinating how big props you can swing with it. I also see that Mega Motor has advertisements for a new geared F3A inrunner in magazines so manufacturers still believe in them.