Devastating Blow-

Contrary to what its description seems to imply, this perk does not actually give you a chance of an instant kill.  The decapitation can only happen if you would have killed the target with the attack anyways, and doesn’t always happen then.  It’s also a purely visual thing, though it can be fun.  More importantly, 25% bonus damage on power attacks really gets you some bang for your buck on them, inasmuch as stamina, being green, is made of dollars.  I don’t know, that’s just how the metaphor goes.  At any rate, a perk well worth it.

Great Critical Charge-

Not as good as Devastating Blow.  Contrary to what it implies, it does not automatically make your charging power attacks critical, nor does it even give them an extra chance to be critical hits.  If your charging power attack is a critical hit, though, it’s horrendously powerful.  Fairly nice if you charge into battle screaming and waving your weapon.

Sweep-

This is really only useful when you’re facing multiple melee foes at once.  Against single targets it means pretty much nothing, and the inadvertent harm you can do yourself by accidentally scything down your idiot melee follower is pretty big.  That said, having this when you’re fighting a number of enemies simultaneously is incredibly useful.  You just have to remember to be careful.  Forgive and forget, Lydia?

Warmaster-

How often do you perform a backwards power attack?  Never if you play in first-person mode.  In third-person.... well, almost never.  I suppose you could charge into battle and then face away just to use this, but really?  As a final perk in a tree, this is a serious letdown.  Sigh.