I only played Portal 2 for like an hour when I first got it, but blasted through it in the past couple days.
I actually found the main-game challenges in Portal 2 (the latter half of the game, once you get to Cave Johnson's area) more challenging than Portal's. I think that was largely due to the open spaces and increased exploration available in Portal 2, so I looked for solutions in the completely wrong places. Net onto the fact that I didn't find any challenge in the original Portal's test chambers. I ran through Portal in one sitting in just under 3 hours, and Portal 2 gave me a few sittings' worth.
Single player campaign did tend to move rather quickly from one idea to the next and there weren't enough chambers that tried to apply everything you'd learned. It was just, here's one mechanic, then here's the next mechanic, then here's the next one, and they're largely isolated, instead of compounded. There's room for much greater complexity there. I also found it funnier than Portal, which was a big plus in my books.
The game was pretty short, but I wouldn't pin this down as a fault: there are no bad bits. Final Fantasy XIII wasn't short. It did have several dozen hours of unrewarding experiences though. Disgaea games are not short. But completion requires hours of grinding. Portal 2 is small, but it is comprised entirely of the juicy bits. Is it worth the full price tag? I'd say more-so: you get a good experience (ymmv) and you didn't have to pay someone to trim all the fat out. This sort of observation is certainly directed more toward those who have much more limited time than those who don't, however, so take it with a grain of salt.
Experimented with co-op mode a bit, but I've only done the first two chapters/chamber-sets there.
Also, I didn't even realize there was an in-game store. So I don't know what people are complaining about that for. It's obviously not intrusive if I can complete the campaign without even being made aware of it.
