Just go watch it already.
This is simultaneously one of the coolest and most beautiful things I have ever been privileged to experience. I'm not ashamed to say that it's brining tears of joy to my eyes. Though to fully understand the significance of the song (“Cara Mia Addio”), you really need to have played Portal 2.
Finding this video was really a wonderful birthday present. Oh, almost forgot, I'm 23 today.