Portal is not only a great game, but at a quality level for others to strive for.
The first thing that strikes you in Portal 2 is the amount of hype you heard about this game. Your friends are drooling over the story, the puzzles, the characters. They aren't smoking crack this time, they know what they are talking about. This game is downright fun.
The puzzles in the game are fun, and like the first one, scale from fairly basic and easy at the beginning to challenging and frustrating at the end. The key to the formula for this and all puzzle games is the "Ah-ha" moment. The amount of times you feel it throughout this game from the story arc to the gameplay is a thoroughly satisfying selling point. This story will satisfy even the most skeptical, but fanboys will be talking about this one for years to come.
Coop doesn't disappoint either, creating some of the most fun moments in the game. The same interesting difficulty scaling and references to the main story make it all the better. This game is not short, but after completing both campaigns in about 15 hours, you will be yearning for more. Luckily, DLC is coming, free of charge. You may be late to the party, but the experience is worth the journey here.