Fantastic- I loved the ending, the boss fight was fun (loved the cores), shooting the moon, of course...
I was extremely happy with GLaDOS deleting Caroline, I'm not going to lie. Even in the context of learning a lesson, actually having it stick would have been completely out of GLaDOS' character, and it was great to see her stick to her rather evil self.
Turret opera- aside from the asinine theories it's fueling, it was amazing, and... well, be honest, who expects a video game to end in an aria?
Chell being 'let go' is interesting, and can satisfy the ending anyone wants. You're free to believe that the world is fine and she'll find civilization and fit in (or at least, have the Cube), or if you prefer... darker endings, she could well return to Aperture, be attacked by the Combine, who knows.
And of course, Want You Gone. So much better than Still Alive.
(Captcha: BOOK Portsks. ... book portals? Book portkeys?)