Really depends. I mean, are they going to keep patching it and updating it, or is it going to be the game as it is out of the box for the rest of my life?
It'd probably be a very open game like Fallout 3, but I'd also prefer it to be very carefully made, creative, and fun like Team Fortress 2. That's asking a bit much, though, isn't it?
In any case, I probably WILL be playing Half Life 2 for the rest of my life since I've never played a game I traverse through so slowly. Seriously, by the time Valve releases Episode 4, I'll be starting Episode 1.