Is it working for you? I have been trying to get it, but once I click the download screen, the screen goes black, like it it didn't load, but I'm not getting an error message. EDIT: Well, I take that one back, it was in my library all along, I just didn't know to go there...
You've only missed about 90 minutes of gaming Admittedly it's pretty unique the first time around, but once you know how to do it a lot of the fun starts to fade and you can speed through in 30 mins.
Well I played it on PC originally then sped through it when The Orange Box came out on 360. It wasn't as fun. The challenges added some extra amusement though, they're trickier than the main game is.
Even so, that first time around is one of the best and most worthwhile in recent PC gaming. IMO replay value is not as important as people think most of the time.
For a game like this replay value isn't important, no, but in less innovative games it really is needed
For example, Modern Warfare 2's campaign. I didn't finish it. The multiplayer, however, has great replay value, despite it being almost as fair as playing baseball with a bendy straw as a bat. Downloading now, but I already beat Portal on the 360. Edit: I guess I was overly optimistic.