I have just finished Portal 2 so here are some thoughts (possible spoilers ahead, but I will try and minimise them).
I really enjoyed it. If you haven't played the first one for some reason things probably won't make that much sense.
Portal had good writing and this second instalment did not disappoint on that front. The dialog was just great; witty, dark and fun. Having Stephen Merchant voice your robot friend added so much more fun to the humour, some nice classic Merchant phrases.
I was a little worried it was going to end too soon, especially as the first one was pretty short, but just when I thought it was over it took a very nice twist and kept going. This back half of the game really added to the backstory of Aperture Science and some nice easter eggs tied things more into the Half Life universe; which did serve as a good reminder that we really need Half Life 2 Episode 3 or just Half Life 3 to carry on the story.
I was sad to see that upon completion I couldn't go back and attempt to speed run the test chambers or try some of them on hard like in the first game; I do love a good speed run (probably thanks to getting Goldeneye cheats). I can see how this might have been a little more difficult to incorporate this time around though.
I have also done a couple of the co-op courses split screen which were loads of fun, more similar to the originals test chambers. Now if only any of my friends on Xbox Live would get the game so I could continue.
Overall the single player is great, a good improvement on the first game, especially in the story department, but it was possibly less fun due to having more of a story. However I think the co-op test chambers make up for this, but I would like to see some more single player testing happen (which appears to be coming in some free DLC this summer!).
Now I better get back to testing, for science.