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.
Here are some other good points about the game. If you buy a PS3 version, you'll get access to its steam copy for free. That means that you'll be able to download it on your mac or pc! Also, the new coop multiplayer mode will be super fun if you have friends who are willing to participate (I suggest buying a headset for this if you don't have one already). Oh and it's much much longer than Portal 1. Single player alone took me over 5 days of on and off playing to complete so your hard-earned money from flipping burgers at McDonalds will definitely be worth investing into this and you'll get over 20 solid hours of gaming pleasure :]
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!