REQUIRES PORTAL 2. A companion mod to the 7-part interview series at Rock Paper Shotgun that focuses on discussing level design and process. You could also call it an "interview mod," I guess. Or maybe it's one of those dreaded art mods? I'll let you decide how to label it, but given the choice, talking about the game rarely matters as much as playing it. So just play it. (This mod was sponsored by Rock Paper Shotgun; built and compiled by Robert Yang; contributors are Dan Pinchbeck, Jack Monahan, Magnar Jenssen, Brendon Chung, Davey Wreden, Ed Key, and Richard Perrin.)
To me this mod is an 8 because the actual project, with all of the beautiful collaboration it entailed, would be worth a 10, no doubt. The commentary that accompanies the levels is also fascinating.
The actual mod from a player's perspective on the other hand is more like a 6 to me, as the latter two chapters I found to be too abstract and lacking in real gameplay, though the first chapter was quite fun.