Originally I bought it because I simply had to play Dead Rising and if I want to play a game bad enough I will purchase a console for it. I almost did the same for a PS3 and MGS4 but had to chose one or the other and DR was the bigger priority. I did end up getting to play MGS4 and was so glad I went the route I did.
Never thought I'd own an MS console but it's been a surprisingly awesome gen. I like the live service, parties and the achievement system much more than PS3's trophy system. I don't have anything against Sony or the PS3, there just isn't anything making me want one and my 360 suits me just fine.
War vet that prefers American-based companies. That would be my only reason, unfortunately. Live gives nothing that means a thing to me, while psn has saved me a bunch by not charging me to play certain games. But no matter what generation it is, from Atari until now, I always look for the next Zelda game. With mods, I like to get games on Steam. Besides, using Steam gets you multiple copies of any game you get. In short, I am not devoted to any company. I play games I like, by anyone on any system. As I always say, the companies do not give a care about me. Why should I care about them?