I enjoyed playing through HL2 and the episodes on the 360, and as mentioned have gone back to get various achievements. However, I have to admit that there's a bit of a strange feeling to doing so.
Basically, it goes like this. It's not really a lie or even an insult to say that characterization in video games is often very primitive. The "people" in them, even the ones you're supposed to relate to beyond the level of shooting them in the head, are simply puppets following scripts. You don't worry about doing something strange, silly, or even counterproductive in the game because it's obvious that no actual person is going to be bothered by it.
Half-Life 2, by contrast, is one of the rare games out there where -- at least in my opinion -- the people feel like, well, people. It's a combination of the way the game never pulls you out of Gordon Freeman's first-person perspective, the high-quality character artwork, facial animation, writing, and voice acting, and the way they react to you, to the environment, and to each other, at least within certain proscribed limits. Alyx Vance, her father Eli, Dr. Kleiner, Barney, and all the rest, I don't think of them as puppets; they're actual characters, like in any good book or movie. I feel genuine affection for them. And given that they feel more real than most folks in video games, and you spend most of the game with at least one of them around, that makes embarking on the absurd challenges required to complete the achievements feel a little strange. Surely Alyx has noticed by now that I've been carrying this garden gnome around for hours. Surely she's noticed that whenever it falls out of the car I stop and go back to get it. Is she going to bring up the subject? Is she going to ask what the hell I think I'm doing wasting time with a garden gnome when we've got to get to White Forest and shut down the Combine portal before the alien meanies swarm through in force and destroy Earth all over again? This could get awkward.
All in all it's a good thing Gordon can't talk because I don't think he'd have a good answer for that.