I don't really care for Achievements, as they just have no 'value' behind them.
That said, there are times ACHIEVEMENTS are awesome,
one example is,
Battlefield Games, They have an 'achievement system', and paired with that, and some kind of 'xp' system, you are able to unlock new weapons/gear.
Uncharted: Drakes Fortune, has the standard 'achievement system' you see in alot of games, and one of the fun parts about it is, every 'achievement' is worth so many 'points' or 'coins', and when you have a certain number of 'coins' you escentially unlock, in Uncharted, different game modes, extra content, ect.
Unless Achievements actually AFFECT the game, I don't give a rats ass about em!
Also, the ONLY flaw in Left4Dead, is the game doesn't Track every stat for every game you have ever played!
I seriously want to know what my accuracy is, the number of head shots I've gotten, how man times I've shot my allies (As well as my accuracy on that

), how many smokers, ect I have killed.
But because the game doesn't keep track of that number, (well it does, just only for that 1 match) the achievements are really 'pointless' to me.
