Fix gear rider forever here. I do both, fixed and geared but fully half is on fix gears. I've made concessions for my age and use multiple gear combos on hilly rides, stopping and flipping the wheel or even unscrewing a cog on very hilly rides. But still, there are a lot of hills I go up in my flat ground gears rather than stop and flip. Likewise going down. I can't say this is a miracle upgrade for my geared riding but it does widen my RPM range a lot and makes the need to upgrade to handlebar accessed shifters not necessary. (I don't race or ride with fast groups or paceline. Not because of my equipment choices but because minor wheel touches are yet another head injury (and I already qualify for NFL "loose brain syndrome").

Biggest physical improvement makes very little difference when all the gears are there but quite a bit in every day life. Much more upper body strength and conditioning than pure geared road ever got me. I love that. I feel better and healthier when I ride fix gears. (Yes, single speed does this to some extent. I rode a single speed about 5 years before racing and my first fix gear ride. But I fell in love with the fix gear first ride and never looked back.)