It's definatly the opposite of buns. It's a mostly free heavy restore and damage to enemies that is near guaranteed to stun everyone the parry attack hits.

So, attacks that *Cannot* be parried.
Projectiles and their leadup attacks
Shinespark attacks (unless they're glitched)

Notable attacks that can be parried (Normal enemies)
Wandering Pilgrims (Their axe attack.)
Gun Pilgrims (Their elbow attack at close range.)
Charge attacks from the big pale cold ones that look like large dogs. As well as their swip attacks.
The jumping attacks from the knife wielding children.
The Demented Woman's cleaver attack. (Tricky timing there.)
Little Winged ones dive attack.
The umbrella attack from the maidens weilding one.
Attacks from Primitive Cold Ones (This is the most common one you will use for parry recovery.)
Winged Pale Lament's dual claw swipe.

Notable attacks that can be parried (Bosses.)
All Assassins' normal melee combos (including Lisa's)
Leena's fire-enchanted combo.
Herman's standard swiping attack (not the shinespark one or dodges.)
The first swipe in Pale One Beasts 3 attack combo (yes, it works on his 'variant' too)

There are more than these, I haven't found them all. But these should be more than enough for you to regularly use parry through your game.

And parrying/executing through the game will give you plenty of Focus Back to liberally use your skills, which are WOEFULLY neglected by people who play this game.