I built my primary box about a year or so ago. My secondary box is about 2 years older than that. All desktops in my house are custom built, and I think there are enough spare parts lying around to build a couple more if I cared enough.
I rarely upgrade, though on occasion more than just the case will survive the next iteration. I tend to buy one rev back from bleeding edge though, and always try to over build as much as possible within my budget.
I dislike working on laptops, both in terms of the internals, and in terms of the little tiny, easily trashed keyboards, so tend to avoid them outside of the workplace. I’ve got a couple lying around, one as a print server, and one needing a rebuild to be given to my wife’s nephew.