The cavalier as written screams Arthurian Knight.
Unfortunately, that's about the only concept that it fits. It just seems to be too specialized from a roleplay perspective.
Added to that, there really isn't any new mechanic that warrants the creation of a new class.
Challenge = repackaged sneak attack
Banners = bardic music variants
Oaths are new, but hardly enough to base a class on.
Its been stated that this class was intended to fill the role of the battlefield leader, and the class as written can do that. But so can the fighter, paladin, cleric, or bard.
I think a better implementation would have been to add feats that make it easier for those classes to fill that role.
A feat to allow a player to pick up a mount. (Similar to whats been proposed for the Cavalier)
Feats to allow a character to focus on battlefield leadership. A rally feat to counter an ally's failed fear save for example.
Even the knightly orders would be better represented as feat chains. (Does the Order of the Shield not admit fighters or paladins?)
I'm probably rambling, but the cavalier just seems to be a too focused concept to be its own class.
Surkin