Actually I use a ThinkPad and an HP Z820.
I work in financial services sector and we serve desktop and web apps. Bar large businesses, we see a LOT of home built and OEM PCs.
I was involved in the family business OEM sector for the best part of 20 years as well from a hardware perspective. There are two types of professional: careless and obsessive. The latter build their own. The distribution I saw was about 40% OEM/home built to 60% big manufacturer.