I said I would never build another PC again but.... just finished up another new one at home and am pretty pleased with the results.
Did it mostly because I had a stack of parts and, that being the case, there remained a (slight) cost advantage. The other motivation was to get a very quiet machine. Everything about this machine is about quiet and it really worked out well. Used a low voltage CPU - less energy, less heat, less fan noise. Used an over-size case - less heat build up, less fan noise. Using a graphics card that is just barely good enough for my needs - less heat, less fan noise, etc.
It is fast (about 3x the performance of my Dell 810 at work) but that isn't crazy fast - wasn't crazy expensive either.
The high performance market seems to be well served either by the boutique vendors or from the high-end machines at big-name vendors. It seems the ultra-quiet (e.g. for a media machine or in a guest bedroom) hasn't gone mainstream yet. So, if you want an extra quiet machine, seems to me that home-built is still an option.