Valve once again delivers an amazing single-player experience with a fun co-op mode.
Portal 2 takes its rightful place alongside the original Portal and the Half-Life series as another masterpiece of a single-player game. The story mode lasted me around 8 hours, stopping to listen to the dialogue as I went, and I'm currently working through the co-op. I can't praise the story enough. Each character is unique and brings a different element of humor, and every bit of dialogue fits them perfectly. The game is also very immersive and good-looking, only taking you out of the action to load the next section.
The puzzles of Portal 2 are all unique and the game builds quite a bit from the basic elements of the first, adding multiple new puzzle elements that can each be used in multiple ways to overcome the game's obstacles.
I also have to give a shout-out to the Portal 2 soundtrack. The music while you're playing, in the various easter eggs throughout the game, and in the ending credits is all good stuff.
The only problem I have with the game is the slightly lower difficulty of the game compared to the original Portal, which makes the lack of challenge maps apparent. There aren't any major twitch or precision platforming requirements, so puzzle-solving is a fairly leisurely endeavor that's more focused on doing the right steps in the right order than it is on executing the steps. There are also no noticeable hooks for official DLC or patch content, which makes me a bit apprehensive as to whether Valve will add more content to the game. In the meantime, though, I look forward to the upcoming release of the Portal 2 modding tools so that the community can build on this fantastic base game.
Edit:: Of course, only a few days after I post this, Valve announces free DLC that is said to include single-player and multiplayer challenge maps. Now that I'm working to get the achievements, it's also worth noting that tracking down many of the achievements in the game are challenges themselves, like breaking the TV monitors. My given score has been adjusted accordingly :-) Portal 2 is easily the best game of the year so far.