According to this article on Game Informer, "Half-Life 2" for the Xbox sold below expectations by a factor of 2 to 3.
I wish I could say that surprised me, but quite honestly, Valve's rabid fanbase may have bit them in the ass on this one.
Last night, I was out game-shopping with my console gamer neighbor. He loves first-person shooters on the console, but can't wrap his head around playing first-person shooters on the PC. They are two completely different animals, and use two completely different skillsets.
He picked up "Half-Life 2" for the Xbox and told me, "I really want to play this, but my buddy said that if I bought it for the Xbox, that I'd be disowned." Digging a bit further, I found out that his friend bought him the original "Half-Life" for the PC just so that he could play it the way it was meant to be played.
I'm going to generalize here, but most PC gamers are very snobbish about the fact that they play PC games. They look at "HL2-Xbox," and see reduced texture resolution, imperfect controls, and lack of multiplayer, and immediately deem it inferior. Console gamers hear about "HL2-Xbox," know it was popular on the PC, so they ask their PC brethren about it. The PC gamers say that the Xbox version is inferior, so the console gamers don't buy it.
Personally, I choose games based off of the platform they are best suited for. Just like I won't usually play platformers or racing games on the PC, I usually prefer my FPS's on the PC. However, that's me. Most console gamers just want to play the game. As PC gamers, we should not let our personal bias against a particular platform for a particular style of game prevent people from playing the game of their choice.
Games are all about making choices. We should choose to not make their choices for them.