Well, so far my Portal 2 experience, other than a ten minute or so venture into the single player for curiosities sake) has been completely on the multiplayer side, which is all test chambers, save for tiny little snippets at the end of each section of test chambers. They are really fun, but I find myself wishing there was a way to bump up the challenge level. It feels like we are figuring them out and breezing our way through them way, way, way too fast. And having two brains on the case instead of just one makes it all that much easier to crack. I am hoping it will surprise us and prove to have a lot more content than I think it will, but I really don't think so. I've heard people say the co-op campaign is six hour long, but we've spent less than three and are already in the fourth set of test chambers, which I think is the final set.
The issues 005 mentioned has kind of been Valves thing these last few years. While their older things like Half-Life were long story driven campaigns, they seem to have fallen into the habit (starting way back with Counter-Strike, actually) of scoping out small developers with promising and original ideas, then buying them out, employing their developers, and allowing them to run with these ideas in ways they previously never would have been able to dream of. Which is actually really cool. This was the case with both Portal and Left 4 Dead, both of which are extremely original and unlike anything else on the market. The quality level is extremely high, and they are extremely pretty and well polished (didn't realize how true that was until playing games like Fallout: New Vegas and Assassin's Creed II, both massive games with tons of longevity, but extremely glitchy and clunky), but the experience ends up feeling short. It is worth it, because the experience of these games is so far beyond anything else on the market, but at the same time it is hard not to look back and feel like you've experienced this fantastic demo over and over and over again and are now ready to experience the real product... which doesn't exist.
I haven't played Portal 2 enough to say if that is the case with it or not.