Some games do it wrong. Call of Duty, with its insta-kill, kind of annoys me. Gears of War melee does enough damage to down you in two hits, but also stuns you temporarily, making it easy to down someone by mashing the B button twice. That's unbalanced.
I like how Halo does it. If you melee someone with shields up, it immediately takes down their shields, but unless you snuck up on them you'll likely get meleed as well, and it becomes a perfectly even fight. If they have no shields, it's an instant kill, but any decent player knows not to stay right next to someone when they have no shields. Only if you sneak up behind someone can you kill a person with full shields. So outside of sneaking up to them (which I think makes sense for an insta-kill), the melee ends up balancing itself out very well, since in any use of the melee you are either killing someone who's weakened or giving yourself an opportunity that, assuming they don't suck, they will have as well; then it's a matter of speed and marksmanship. That's pretty perfect.