Amnesia is actually the easiest to do, all you need to do is start the game and make a new profile called "Casbah" then put the password "troubled memories" into the window that pops up.
Unfortunately Amnesia is the only game I own, i'm not going to spend £27 on a pack that I will only play once to help unlock Portal 2 then never touch again.
There are 2 of them. You get the second for completing the Justine DLC 100%.
Honestly, if i only owned 1 game i'd get the pack because there are some damn good games in it. As it stands i own 6 of them, the combined price of the rest is £26.93...
Audiosurf is great but i've not really been on it since i got Beat Hazard.
Reckless is a good bit of fun.
Cogs is a great puzzle game.
Killing Floor is... well killing floor.
Defence Grid is 1 of the most well balanced Tower Defence games you will ever play.
Super Meat Boy was good fun on the xbox demo i played and it would definitely be worth getting.