I don´t think it is OP, but Paladin is a very good mage. I think the best new addition after necro.
He has access to the best cards in war school (ballista, ballad of courage, harshforge plate...). He summons more than decent creatures (I always thought holy creatures where the best medium size buddies) and has both a very good attack ability (challenge) and a quite decent long term one (auras).
He can go on attacking anyone because he can heal, and that is a good point in a damage race. He hits fast and hard with ballad of courage etc, and has access to quite a good range of equipment (sword is awesome, and I can´t decide which armor is best).
Has a good chance against one of the best mages nowadays (necro). And he can build a versatile book.
But: The spawnpoint is not great I think and he somehow needs it, and versatility always comes with a flaw: jack of all trades, master of none. He almost masters rush attack, but he lacks the +1 dice on other mages... he can hold a position while his ballista and royal archer shoot the enemy, but he lacks himself the arguments to shoot (no good inschool ranged spells, not likely to be a better archer than priest).
I personally think he is one of the 5 best mages, but if Paladin is overpowered, what do we say about Forcemaster?