I started main'ing Galio about 2 weeks before the nerf-bat got applied to him.
Galio's current state is as follows:
1) Two AoE spells on high cooldown. Good for clearing waves of minions, but negligible vs champions, as in you get one relatively ignorable burst in a teamfight, and are pretty useless afterwards. If you Q-E-R, you might get a second burst in afterwards, but don't count on it.
2) Ult is nowhere near as damaging (or disabling) as other tanks unless you go straight caster, and at that point, you don't have enough survivability after your ult is finished.
3) MR-to-AP ability means you are a "hard" counter to AP-heavy teams. However, most AP champs are ranged, meaning trying to ult the enemy will result in the enemies being on the edge of your ult (easy to escape w/ MercTreads) or the enemies will be hiding behind the DPS/Tank. You have to be in the middle or back of the enemy team to get the most effect off of the ult, giving you heavy dependence on Flash for positioning.
4) He is extremely item dependent. He farms well, especially in the solo/2v1 lane, but cannot put pressure on the enemy during laning (due to long cd and relatively low damage spells, and melee AA range). You will have to gear AP heavy to do any damage, or gear tank (and have ignorable damage). Only with a full build can you be both at the same time (Frozen heart, RoA, AA Staff, BV, AbysScept for example).
He is a counter-pick, not a main character anymore. Only get him if the entire other team is AP-based and you want to stack MR and still have damage.