I'm still waiting for my copy to arrive, saved a few £ by ordering it elsewhere but couldn't wait for it to get really cheap. I have high hopes for this game, I got a lot of hours play out of portal even after I finished the story trying to get good times on the advanced maps and get the medals (there's one I just couldn't do quickly enough or in a few enough steps). I hope I get as much fun from this game. The comments sound promising.
There aren't any of the time trials or advanced test chambers yet, but there is co op, and I wouldn't be surprised to see valve introduce them at a later date in the form of free DLC (valve tends to release free DLC).