The chance of getting at least one specific buff from galvadon is ~83%, so if there's something you need galvadon get you'll probably get it. So if there is a buff you need you'll most likely get it. Stuff like stealth and shroud overlap a little so if you want at least either defensive buff it's 99%.
As far as the minion itself is concerned he offers some solid consistency, but even with all of the defensive buffs in the world he still can die to things like devolve, twisting nether,king mosh+ww, volcano(unless you get +3 health 3+ times or 3+divine shield), pyro+equality(even with divine shield, they just need one more spell), brawl can all deal with it. Probably more I'm forgetting that are practical. I have a hard time believing you can get him out turn 5 and every turn after that his risk of being removed increases.
When they see you playing a paladin deck they will save removal for your galvadon because that's your only win condition and they know it, or they will rush you. The rest of your deck will struggle to keep up and when you finally get your check card you need to hope they don't have the answers. Aggro decks will probably win the race since paladin early game minions+cycle is ass, and all meta control decks have reactive direct answers except maybe priest or mage. Yet even still priest has mass dispel which ain't good but might be a tech, and mage has stall after stall after stall.
So galvadon is good, probably would see play at even 9-10 mana in paladin, but the quest is probably garbage and the deck along with it.