I have 48, missing most of crash course and survivor ones beyond the super easy play this once and you'll get it type of thing. The ones im missing from the original bunch are the expert ones, no friendly fire, take no dmg in finale and two or so of the kill x number of zombies.
When achievements first came out I thought of them as silly things to make certain people buy the game just for the gamerscore/e-peen, but I've grown to like most of them now, for instance when I was playing tank for one of the first times, I was doing really well and managed to knock down all 4 and was feeling all chuffed then a few seconds later up popped the achievement and I was thinking 'Yay go me'. Other ones like use only pistols or no healing are nice as well when your playing with friends and want to add an extra challenge to the game.
Well what im trying to say I like them when they reward (well recognise) you for doing well or give you that extra bit of challenge to help keep a game fresh.
EDIT: RAR another one line post that turned into a small rant yay