Let me start out by saying, I haven't finished the game yet. I'm currently on the part where white gel is first introduced.
With that out of the way, I'm going to say, this might just be the best game I have ever played. I was a pretty big fan of Portal the first. Its atmosphere tottered between quirky science humor, and intense puzzles, and this game is that, times about one hundred. Between Wheatley's sense of "wit" and glad0s'... unusual new mood, it all just fits. The dialogue is seamless and flows brilliantly.
The puzzles themselves are a step beyond. A few of them so far have actually taken me around 15 minutes to complete (which, if you knew me, is an eternity). Every new aspect feels... well... new. From the first time you get light bridges, to the first aerial faith plate, to the first time you see blue gel. It all feels new, and you feel like you're actually learning how to do something instead of just fucking around for a solution.
I'll type more on this later. A quick final note: Ken, if that business man keeps following you, I've got your back. I've been itching for a fight lately anyway...