It depends on the type of gamer you are. Here, let me break it down (full disclosure, I think Portal 2 was one of the best games to come out in the last decade).
The Puzzles
Portal has always been, and will always be, about the puzzles in the game. The concept of thinking with portals was an amazingly unique way to view puzzle solving, and mixed with physics puzzles it was a great experience - though not the easiest, to be sure. One thing that Portal 2 does exceptionally well is keep the puzzles just challenging enough to ensure that you (the test subject) stay entertained without getting too frustrated, while also making sure that they're not too easy to be boringly simple.
There are a lot of complaints from the most experienced of Portal players that Portal 2 was "too short" or "too easy", but after spending the last 4 years playing highly advanced testing levels they expected that Valve would have made the levels in Portal 2 equally challenging - and this is where they were disappointed. Valve made sure that new players would be able to pick up the game and be able to learn the Portal gun from the very beginning, just like we did in the first game (though if you haven't played the first game, I highly recommend it both for the amazing puzzles, the awesome humor, and the overall great experience that the game gives). Don't feel intimidated by the fact that this is a sequel to a puzzle game, you get to learn the mechanics at your own pace and you'll have fun with it.
The Story
Valve did a really great job making this game accessible to those who never played the first Portal game, and the story is a shining example of just this. The story of the first game isn't spewed out at you in the first five minutes of the game. It's not even really _told_ to you. Over the course of the first "act" of the game you are steadily given little hints toward what happened in the first Portal game without ever being told exact things (well, for the most part). It's up to your imagination to decide exactly how things went down.
Speaking of the story, Portal 2 has the best writing I have ever seen in a video game, and one of the most compelling stories as well. The humor is even better than in the first Portal (which is saying a whole lot, since one of Portal's strongest aspects was its ROFLMAO-caliber dark humor) and the game introduces two new characters for new and different (and still completely hilarious) dark humor. I can guarantee that you will laugh at this game, and I have no hesitation in saying so objectively. This game IS funny. You WILL laugh.
Difficulty
Even if you're not someone who plays puzzle games a lot, don't worry. All of the different mechanics are taught to you in Portal's own unique way of doing it - the ridiculously funny AI will make a joke about you that hints as to what you have to do, you're given a ridiculously simple type of puzzle involving the new mechanic to solve, and then the next test chambers progressively build on those mechanics, incorporating them with other things you've learned and the like. For instance, in one of the earliest test chambers, you have to use a laser to make a platform move; in a later puzzle (much, much later) you use portals to make a laser make a platform move, then use portals to make your way onto the platform, then use the laser to open a door, then use portals to get to the door. If you had been given the first test at the beginning of the game, you'd be more lost than a hockey player in Brazil, but you're slowly learning new things as the game progresses, so by the time you get to that puzzle you know what you have to do.
In the developer commentary for the game (which you unlock after beating the game) you hear that Valve spent a LOT of time playtesting the game to make sure it was accessible to players of all skill levels, and across all consoles. The controls are more intuitive, the mechanics are more forgiving and guiding than in the first Portal game, and overall the game is more playable to people on consoles (there were some levels that took me hours to do correctly on PS3 in the first Portal that took me mere minutes on PC; the difference in Portal 2 is almost non-existent).
No matter what level of skill or experience you have coming into Portal 2, you'll never feel lost or overwhelmed - and if you do, just keep at it and eventually you'll reach that AHA! moment where everything falls into place and you feel like the smartest person on the planet.
The Final Word
Portal 2's single-player experience is bar-none one of the best in recent memory. There's plenty of puzzles of varying and increasing difficulty for all skill levels, a number of new and refined mechanics to make the gameplay more fun for the experts and more impressive for the newcomer, and the most hilarious writing in a video game that I personally have ever experienced. It's a refreshing break from the first-person shooter games that have dominated the market, and it's the kind of game that fully deserves the high praise it's been given.
I haven't even mentioned the co-op mode, which has even more challenging puzzles than the single player, and a really well-integrated mode of communication to let your partner know exactly what to do, even if you're not there with him/her. Additional content in the form of DLC has been promised for this summer, and you can bet that means new test chambers to prolong the life of your game. And if you have the PC version (which comes free with the PS3 version, by the way) you can download and play custom maps from the Portal community - and there are already dozens of challenging puzzles.