Technically the game looks good and is fun to play, you really get a sense of achievement when solving some of the harder problems, however none are really that hard. I guess if you played Portal 1, you will find it easier because you're already used to the portal way of thinking.
They have added some new type of technological stuff, like some light bridges, fields and gels which give you more possibilities, however these are not really big game changers. They add variety and it's cool but they never really get used in combination to make you think out of the box. Typically, you just get to deal with 1 or 2 elements at a time, so it's a bit of a let down, they could have pushed the difficulty a bit more.
There are some really nice sound tracks in game, like in other Valve games the music artist(s) deliver super catchy tunes. With one exception the end-track, read below.
Slightly disappointing, even though I didn't rush, it only took me 6 hours to complete the single player game. For a $50 game, that felt like being shortchanged, I would have hoped for two to three times that playtime for that price. But that's a general trend in gaming industry, you get less for your money every year.
The story feels weak and predictable compared to Portal 1, but well there's a simple story at least. But I wish it would be a bit more surprising.
The humor felt forced and bitter, to me it felt like the team who did this tried too hard to be funny and ended up failing. In other words, it felt like a job, a mechanical forced way to pack jokes assembled by a group self-centered story-specialist sipping coffee for hours in meetings (anyone who ever worked in a software company will get the picture). In contrast, the humor in portal 1 came progressively and was extremely funny (may be it was the work of smaller team).
Ok, the first part with the hyper active "ADSD (Attention Deficit Syndrome)" computer core (that round thingy that keeps on talking stupidly with an english accent) is verging on being obnoxious. This part is so lame and feels so forced, it almost made me close the game or at least to play it without sound. Fortunately this only last for 30% of the game, after that things settle down. To put an image it feels like those Walt Disney 3D animations where the characters are over-scenarized and keep talking.
The ending song, I realize they tried to do a superb song like for Portal 1 (which was so catchy and addictive and fitted the story so well) but let's be honest, they failed miserably. To their defense, it's hard to do a hit song on-demand, even rock stars can't do that. So I don't want to be overly harsh on them, just honest.