Granted, most people play whatever the pros play... but why aren't even the pros playing Galio? He counters quite a few midlaners (especially Syndra), he's tough to gank, he's great in team fights, his damage/AoE is pretty solid, he's tanky and can force/punish engages, he can give peels. Sooo, where are all the Galio's? Haha.
For an enjoyable laning experience, try taking Galio as Support with a Kalista. Nothing quite like being launched into their team so you can ultimate everyone with no counterplay.
[slayer-pantheon-thumbs]
Mana issues.
tl;dr Ask yourself, do you want to play 
a shadow ninja? 
or some statue guy?
personally i love galio ... i play him as a counter to an ap comp at support.
but in pro play he is pretty one dimensional and easily played around
he is in need of an update/rework cuz outside of flash ults he provides little
Also rework incoming. Soo playrate usually drops off
