No hard feelings
For a puzzle game, Portal 2 actually has a remarkably strong story. The basic plot is solid (I won't give anything away because it's well worth the surprise of going through it yourself) but the background and characters are the aspects that really make the game's setting and context strong. As you progress through the game, you're given a lot of background about Aperture Science that is actually pretty intriguing, and it all happens as you continue to discover new level design elements and solve more great puzzles. GLaDOS (who was (supposedly) killed by the main character at the end of the first game) returns with even darker, wittier humor than before, and this time another robot character named Wheatley enters the picture. He proves to be just as hilarious as GLaDOS, albeit in a different sort of way, and the voice acting for him is pretty much perfect. These characters actually are a part of what will keep you coming back to this game, because their constant bursts of humor and commentary are a major part of what keeps Portal 2 interesting.
Gameplay and level design reign supreme in Portal 2, though; the game thrives in these areas more than any others. Once again you are given a portal gun that can shoot two invisibly linked portals, but this time around a wide array of extra elements have been added to the puzzles to keep them creative, complex, and challenging. Propulsion and repulsion gels are especially interesting: repulsion gel makes you bounce off of it, whether it's splattered on the floor or on the wall, while you run faster if the floor beneath you is covered in propulsion gel. These two elements alone add incredible depth to the game, and when you add in other twists like Arial Faith Plates that fling you into the air and Excursion Funnels that carry you in a particular direction within a wide beam of some kind, there's plenty of depth and challenge at every turn. Of course, the levels are designed to utilize all of these mechanics to their maximum potential, often incorporating many of them at the same time, so the game continues to feel fresh right to the end.
Portal 2 is much longer than its predecessor, clocking in at about 8-10 hours of single player from start to finish. It's not a particularly lengthy game, but once you finish single player you can always find a friend to play through the brilliantly executed co-op mode with you. Though I have not played co-op for more than a couple hours, even that short time has given me a great taste of the complexity and creativity it presents. Using two separate characters, each with their own regularly functioning portal gun, works perfectly and presents plenty of fantastic obstacles and challenges to overcome.
If you enjoyed the first Portal game, there's really no reason for you to not get Portal 2. It's bigger and better in every way possible with innovative gameplay mechanics, challenging (but never impossible) stages, and a strong co-op mode. There really aren't any flaws to speak of, either. So really all I want to say is that Portal 2 is a fantastic game, and you should get it if you're even remotely interested; if you play it then you're in for a real treat.