It is nothing short of a disgrace that this games overall rating has been lowered to 4 stars just because some people expect to play forever.
Playing it 'till you complete it on the first day is just stupid, jesus, go outside once in a while and you'll find games last a lot longer!
This game is a masterpiece, from it's originality and uniqueness (games have now copied it however..), to it's brilliant voice acting, from its level design to it's truly great story.
I have yet to play a game that I have so thoroughly enjoyed as I have this, and it is made even better when you have a friend to play Co-op with.
Portal 2 is a sensational game, which if you look deeper than just how to get out of the room you are in, and in to its story and characters, you will never forget.
(plus for those who complain about the games length - Valve are releasing public tools so everyone can easily make their own levels and post them online for downloading, the game will never end!)
5 stars well earned!