I have the Enthoo Mini XL, I had planned on a custom water cooled system but I am thinking of building a second system or NAS in there after Xmas.
I usually have a couple of PC's running, it's handy when working with Linux configs, playing with server OS's and even a general PC at hand as you game, or if your kids or partner want to play the same game with you.
But the multiple cases, ATX and Micro ATX bug me. The secondary system can easily be Mini ITX, as for the SFX PSU, well I do not think it looks that intrusive, and if it can be fully modular with braided custom cables it will look no worse than some systems with a PSU at the bottom.
My main worry would be how close the bottom mounted SFX PSU gets to any future GPU's.