1. Install isn't painful -- less user interaction required. That's nice.
2. Hardware support is very spotty at this point. Windows XP drivers mostly work, but I was installing this on a NForce2 motherboard, and NVidia doesn't provide a functional driver for the chipset network adapter. Luckily that motherboard has a second controller, for which the XP driver got me networking.
3. Audio Hardware -- the motherboard audio worked once I installed the XP drivers for it. The XP drivers for the EMU 0404 card in that machine worked as well.
4. 'Glass' -- the graphics card in that box isn't that old, and it's a Radeon, but not one apparently supported by the ATI Longhorn drivers. I may pick up a different card just to play with the new UI stuff.
I haven't managed to crash it, which is good because I break everything. Subjectively, it feels very solid, and no slower than XP. It really feels like XP, with a few little bits changed here and there. I know there's a lot of change under the hood though, that could be the basis for cool applications. But it is a lot more familar than I would have thought.