Just like in Portal 1, there's an interesting twist in the storyline and I'm sure you guys will enjoy it, but I find that it wasn't as surprising as what happened in Portal 1. Nonetheless, the characters are amazing and I found myself laughing many times during the game because of Wheatley's funny comments (the thing that will follow you for a good half of the game).
From a visual aspect, the game has been greatly improved in comparison to the prequel and if you have a nice video card you will definitely take advantage of it (not to mention that the main character has smoother curves ;D).
As far as gameplay and puzzle-solving goes, you'll be surprised at all the additions Valve has prepared for us. If you think you got pretty good at Portal 1 and that Portal 2 will be a piece of cake? Well you're wrong! There are different kinds of new physics to consider (spoiler: gravity beams and catapults) and you'll need to do some serious 3D thinking for some of the puzzles. You're probably familiar with the typical situation when a good game/movie is made and later its sequel just takes advantage of its success but gives you almost nothing new. Rest assured, Portal 2 is NOT that kind of game.
In summary, this is an amazing game and it has almost no flaws. Moreover, whether you prefer RPGs, FPS or RTS, it doesn't matter: you'll like it anyway!