Just finished this and think it's one of my top 3 fave games... even better than the first Portal and not at all dumbed down. I'm normally the first to moan about a game being dumbed down, but not this. Everything is better than the first game... apart from the ending credits song.
If Half-Life 3 can combine the Portal gun with the puzzles and all the other elements of Half-Life it could well be the most perfect game ever.
Dont think anyone has mentioned this either, but the game has excellent animations!
The Source engine is showing it's age though, but still looks good and this is the best looking Source game so far. Dont know why people are surprised it runs well though, all Source games lack lots of dynamic lighting, which is very demanding. They do have some dynamic lights, just not loads like most new games. A lot the lighting is 'baked' onto textures (it's a flat 2D shadow texture over the original texture), its not real-time lighting. And obviously the textures themselfs are lower res than a lot of other new PC games, and polygon counts are not high - lots of flat walls and so on. Yet people give Source games credit for running well... well, thats because of the reasons above, not the engine being amazing. Most other games wouldn't get away with doing these types of things without the game looking really poor, but Valve can because they highly polish everything, do quality level design, animations, and clever ideas, which makes a limited engine still look decent.
And i dont know how anyone can do this game in around 6 hours or less on first play without cheating, rushing like mad, or googling how to do areas. Dont care what they say, they're a lier and it's just for there e-peen, when in reality they just look like a stupid immature kid to everyone else.