If some non-techies just wants a cheap PC for word processing, browsing the web, sending emails, etc I usually just send them to Dell. The economies of scale mean you can get a lot of PC for your money, covered by warrenty and with OS and apps included.
If someone is a geek or has a specific requirement, I'll build one to match their specific requirements and budget, getting the biggest bang for the buck and using quality components with an eye to futureproofing, noise, heat, and expandability. This will work out more expensive than Dell, but the components will be of higher quality without compromises being made as when you buy a whole PC off the shelf.
For myself I'll always build one, for the same reasons as the paragraph above. I may spend more, but I'll get exactly what I want. I've found that building better computers and spending more at the beginning actually works out cheaper in the long run, as you end up with a PC that has a longer useful life, or one that can be upgraded with minimal cost and components. Plus I actually enjoy doing it. Even reading the reviews and choosing your components can be fun.