Yes, I and probably most of the people on this forum have both FG and geared road bikes. Basically, a geared bike always trumps a FG on long rides on varied terrain and where wind is involved. If it's flat and windless, it really doesn't matter. As to stamina, endurance etc., I really don't know, since I ride my FG and geared bikes differently, so I think they really complement each other and both are beneficial, but in different ways.