As with Great Critical Charge, this is not as good as the alternate, Savage Strike, since it relies on you scoring a critical hit rather than providing a chance at one. This -is- better for a dual-wielder than for a weapon-and-shield fighter, since the dual-wield power attack uses both weapons and thus has two chances to get a critical strike, so it is slightly more effective than Great Critical Charge... IF you are dual-wielding anyway. And charge-attack a lot. The rest of the time, just take Savage Strike anyways.
Speaking of Savage Strike, this perk is actually especially nice for those using one-handed weapons. Shield users have sacrificed a pretty hefty chunk of damage for the ability to block more effectively and use shield perks, and while the fact that blocking and power attacking both draw on your Stamina is a bit annoying, this can go a good way towards both lightening the strain on your Stamina pool and making up for the damage discrepancy. It’s a pity the decapitation can only happen if you would have killed what you were hitting anyways, but it’s really the 25% boost to power attack damage that you care about to begin with. As far as dual-wielders are concerned, that’s a huge damage boost for power attacks, laying things out flat even faster than you were doing before. What’s not to like?
Dual wielders, behold one of your best friends ever. This stacks with the other power attack boosters, and is effectively about the same as having a third weapon when you power attack. That’s a massive damage bonus, and the only thing that doesn’t make it as good as Armsman is that it has no effect on your regular attacks. This can turn your power attack into a thoroughly deadly opener, though, making whether or not you land that starting blow a critical point in any battle. Should you have grabbed Critical Charge, this can make those rushing critical power attacks absolutely silly.
Once again, how often do you do a backwards power attack? Yeah, I thought so. Extremely powerful when it goes off, the requirements on this are sadly such that you’ll pretty much never use it unless you deliberately show your back to an enemy. You -always- have something better to spend your perk on.