My desktop experience so far was that Win 8 "RP" booted quick as long as I only had SSD or maybe single HDD on the system. After plugging in all my SATA HDD's, win 7 boots about as fast, the Win 8 boot slowed down a good bit. I also didn't like the new OS selector which takes a good bit to load and then when you select Windows 8 it's already loaded. Fortunately when I went to choose to boot to Win 7 it switched back to Win 7 boot menu and hasn't switched back.
I tried installing few games to both clean 7 and clean 8 RP and in 8 RP the installers exhibited some bugs. Not a good sample size but I suspect that if in Vista some drivers stopped working, in Win 8 it will be some old apps stop working more or less subtly.
I was intending to install Win 8 RP to my old laptop but apparently newer cpu with NX is required.