Well, I've completed the single player campaign last night, and all I can say is that it was freaking epic. The story and the humour are just amazing, plus I think the campaign was more than decent in length (it sure as hell felt a lot longer than the official "6 Hour" statement to me) and is very satisfying.
Of course there's also the co-op campaign, but unfortunately I won't be playing that since I don't have Live or an extra controller. (T^T)
Still, is great to know the game has a great deal of content.
But if I had to complain about something, I guess it would be that the puzzles didn't seemed that though to me. And obviously the end song is nowhere near as great or charming as the original's "Still Alive". But let's face it, we all knew it was going to be hard, or more like impossible, to top it.
All in all, amazing game. And I don't say this often when it comes to current gen games. This is the first game in a while that made me want to replay it right away after clearing it, and I'm actually doing it for a change!
I'm sure there'll be lots of DLC coming for this game. So I really hope they release a Game of The Year Edition/Portal Box, or something along that line for those of us without Live.
Anyway, I just have two final words for this game : Must-Buy.
Oh, and by the way... On an interesting note, I noticed the credits mentioned the song Still Alive which I don't remember hearing in the game. I really hope they put it on hidden around somewhere, and of course I hope it's on the single player campaign.