Achievements are gimmicks that create artificial replay value, nothing more.
I don't understand playing a game (once through or multiple times) solely for the sake of achievements. Do it because the game is fun, not because you get something in return. And I really don't get how some people won't play (or replay) a game just because they think the achievement system sucks.
Achievements are totally separate from the game - they're an extra.
Not only that, but I agree with Spo: I'd much rather have an unlockable that actually gives you something you can enjoy rather than a pointless number increase.

