Well here we are again. This is Portal 2. I STILL LOVE THIS GAME! So it's more of the same. There is a larger cast of characters with one more who is Wheatly. Not going to go into Wheatly in fear of spoilers but he is a personality core. So there are more puzzles in this game along with the ability to make and upload your own test chambers to the Steam workshop. Also with this update there is CO-OP GAMEPLAY! So I have not finished the co-op gameplay, but so far it is fun with a friend. As with the first one there the soundtrack is included for free and also the co-op is included free too. So GLaDOS is still here and puts you through some tests. Not going to go any further for, of course, spoilers. I am in love with these games. I just wish Valve could count to three. This one includes some bonus challenge maps like the first one, but there are no achivements for those maps. This one does include some more easter eggs and achievements for those achievement hunters. So that's really it.
Buy on Steam for $20 on PC, Mac, and Linux: http://store.steampowered.com/app/620/. As with the first Portal got it on sale as a bundle.