Note: I am a DM, not a player.
I like power attack simply because it manages to provide nearly the same kind of tension as a save or die spell without being nearly as potentially game wrecking. When in the hands of the "Barbarian Lunatic of the Week", I can max out the power attack, and still have a reasonable chance to hit due to the strength bonuses. When it does hit, I do very scary amounts of damage, but generally not enough to one shot a PC to -10 hp or less.
With the same opponent not power attacking, I can probably hit more often, and statistically I may do more damage that way. But the lifespan of the typical NPC opponent is pretty short. Scoring 2 or 3 hits for 1d10+5 at +11 to attack rolls is a hard hit, but not distressing to the front line types. They are laying in just as much damage, and they have a cleric to fall back on, and a spell caster who will blast the guy with some magic missiles. A good fight, but not memorable.
But on a maxed power attack for 1d10+17 at +5 to hit, that changes the situation. I may only hit once instead of 4 times. But I now have the players attention, and they know they are one critical hit away from biting it.
As a DM, power attack lets me create more tension in a combat encounter without dropping more loot. As a player, Power Attack lets me treat stronger opponents as fodder. I lose to hit due to the attack penalty, but I am more likely to get use out of Cleave if I do use it.
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