I used a out-of-case board on my first build, An AMD 1 gig (Im planning to upgrade it soon).
On the next two builds I used IBM NetVistas, with 2.4 gig P4. I got these machines cheap off the net, and I like em a lot!.
My last build, due to the budget I was given, I used another bought off the net, P3 997 mhz PC (cant remember the name of it).
I definitely like using a PC in the case.
The first I built looked really cool, because it looked so "out of the ordinary" on the inside. But we had a Christmas party and the arcade was the focal point of attention, but NO ONE looked inside it.
And mounting a PC case is a lot easier than mounting a board, power supply, hard drive, etc.
So I went with in-the-case PC's from then on out, and probably will next time, unless I come across a super cheap deal on a MB CPU mem combo.