If you were a fan of the original Portal, you don't need me to tell you to run out and purchase Portal 2 immediately. If, however, you've not yet experienced the wonders of the Portal universe, well, I've got good news: You're going to love it.
And this comes from someone who's a diehard shooter fan: Call of Duty and, well, just Call of Duty pretty much. In the Portal games, there's no shooting per se, but what there is, is some amazing puzzle solving, a great plot (especially in Portal 2), and, surprisingly for the age of the game engine, some very impressive graphics and sound. This is a great, great game, better in many ways than the original and certainly quite a bit longer.
If you missed out on the first one, go back and do that first. In the first, very short game, you play Chell, and over time you come to understand the world in which you live, as a test participant for Aperture Science. You're guided through a series of tests by a computer AI named GLaDOS that, it becomes increasingly clear over time, is insane and trying to kill you. Using a portal gun, you solve a series of puzzles and, eventually, escape from GLaDOS and Aperture Science.
There are promises of cake, incredible humor and great voice actors, and if the original Portal has a problem, it's that it's just too short. Portal was made available originally only via a strange Xbox 360 package called Orange Box, which consists of other Valve games like Half-Life 2 (and HL2 Episode 1 and 2) as well as Team Fortress 2. If you're a fan of the HL games, and who isn't, you'll appreciate the many HL references in Portal, including a rivalry between Aperture Science and HL's Black Mesa. (Chell also looks an awful lot like HL 2's Alyx.)
Portal 2 opens with you, Chell, trapped back in Aperture Science--apparently your escape at the end of part 1 wasn't so final after all--and awoken years later after being kept in stasis. The Aperture Science labs you encounter have gone to seed and often don't work correctly, and you discover that GLaDOS is still around, though also in her own state of (temporary) hibernation. Eventually, of course, she wakes up and all hell breaks loose.
Compared to the first game, Portal 2 is, as I noted, longer, but it's also less single-mindedly focused on repetitive tests. Here, the story tellers get to stretch their abilities a bit, and the Portal universe is opened up wide, with a very nice retelling of the history of the company and how GLaDOS came to be. You see it coming a mile away, but it's a great story.
There's also a new character, Wheatley, played by British comedian Stephen Merchant. He's likeable at first, but his delivery is straight out of Ricky Gervais' "The Office" character, right down to the same lilting vocal style, and it gets tiring over the course of the game. (Surprise, surprise: Merchant was a co-writer on "The Office" with Gervais.)
Some have complained that the puzzles in Portal 2 aren't as hard as those in the first game. I'm not sure that's true, and I had a hard time with some of them. (Remember, I do play Call of Duty a lot, so puzzle solving isn't my forte.) I think the new game is just more approachable than the first game, and is certainly more of a game in that you can play it for quite some time. So far I've only finished the single player experience, but there's a co-op mode as well, and I'm told it's even better.
Imagine that. Portal 2 is highly recommended.