I see a good deal of people in these forums posting about selling machines they just bought a month ago in order to upgrade to the newest thing that Apple just released. So, I thought I'd ask the question: how long do you use a machine as your main computer? I've been getting 4-ish years out of mine, but probably could have made it longer if I had taken better care of them. I'm a year and a half into my latest (a PowerBook), and I don't see myself upgrading for at least another year (and if it's that soon, it will be because my requirements have changed). Others?