Well after 10-12 hours of playing, I beat it. It was AWESOME! Now just because I beat the main story doesn't mean the game is done. I still have some very cryptic achievements to figure out and then there is the co-op. I haven't even touched that yet.
At any rate, this is just as fantastic as the first Portal and maybe better since it is longer and more involved. There are nifty new things in this like the specialty gels we've all heard about. I won't give anything away but let's just say there's a twist.
The dialogue is just as humorous and well thought out. The voice acting is spot on and GLaDOS is just as insulting.
All and all, a must buy. If you like puzzles, you love this even more.