I went to pick up the PS3 version so I could get the free steam download (and then sell the PS3 copy back), but they were sold out until wednesday.
you'll enjoy it. its got more and funnier humor. Stephen Merchant and J.K. Simmons' contributions add a lot more depth to 2 over 1. Still. even with all the new objects that really make the puzzles interesting - i didn't find the puzzles nearly as challenging.
the coop is a little more challenging though since when you look at the puzzles, you have to think about how to solve it with 2 people and 4 portals. when we beat that though, unless some of the DLC is new levels, i dont see this having much replay value.