very good overall but make the base stances incorporate a penalty as well. E.G.

Defensive stance: +2 AC, +2 Reflex Saves, +2 Grapple, -2 attack.
Offensive Stance: +2 attack, +2 Damage, +1 crit threat range, -2 AC.

That sort of thing. Obviously the more advanced Stances don't need a disadvantage.

Also, this PrC has given me ideas for Axe Masters, Spear Masters, etc.