I managed to get it to run on my 3500+ Newcastle (I think that's the right name).
It worked. It loaded the desktop and at best I could do what most people do at the Apple Stores: Poke around, look at things, web surf.
There is a technique that allows you to boot native. It takes a lot of fiddling but it does work. If you don't have the right features on the CPU (I can't remember the name. It wasn't on the Newcastles, but was on the next revision), lots of the interesting stuff doesn't work. Things like iTunes don't work without it.
The really interesting thing I found was it was transplantable. I could take the drive from an NForce3 (Shuttle SN95G5 v1), stick it in a Intel i865 Pentium 4 and the thing would still work. Windows doesn't let you do that (at least, it doesn't let me
Keep in mind too, it was a hack to get it to work. And everytime Apple updates the OS, it usually patches the hole that allowed it to work in the first place. Then there's this gap where the guy that got it to work in the first place, has to find a new way to hack around the patch and allow it to work on non Apple branded hardware. It might be simple, or it might take weeks. You might even find after a month that the guy gives up and says it's impossible. You're then left with software that's out of date. It's a losing battle.
To answer your other questions:
Ease of install: You have to fiddle a lot with it, but it's pretty straight forward and there are step by step instructions floating about.
VMWare: You can, but you can also download a pre-made VMWare image and burn it to Disc. You can then use it to "Burn" a hard drive with the VMWare Image. This is the way you use to make it run native (non-emulation).
Speed: Fast. Very fast compared to the G4 at the time.
Multi Core: Didn't have one to try. (The Pentium 4 only had a virtual 2nd CPU).
Certain Intel CPUs: No, you can use any X86 CPU. They all work with the basic OS (including web surfing), but unless you have that special feature of the newer CPU (at least, newer than Newcastle), iTunes, and bunch of other stuff doesn't work (I don't have a list).
I'd sell the Shuttle, a look at getting a high end Apple Tower or Laptop. That's what I'm doing with my SN95G5.
It's funny you know, if I had never ran pirate OS X-x86 on my PC, I would never have switched to Apple machines.