Achievements in some games are ideal, for example Team Fortress 2; the achievement model adds a new dimension to an - admittedly - already brilliant game. It gives you more incentive to play with and master the different classes available.

I haven't tried Episode 2 yet, but I would agree wholeheartedly that I wouldn't want anything to detract from my immersion in Freeman's world.

Perhaps in a game like Half Life Episode 2, the achievements should only be opened up once the game has been completed once, to give it more replayability and avoiding the danger of such a brilliant, dark and involving concept becoming "just a game".