My whole point is that the XPC and the Mini are two different markets. Completely.
A person looking at an XPC is not likely to be looking at a Mini. For one thing, the XPC is a barebones PC, which implies some building. Or you could buy one of their prebuilt ones at a helluva premium. Either way, you can configure it to compete with some of the top of the line gaming rigs or HTPCs.
The Mini is what it is. It's a nice little machine for doing the basics and learning how to use Mac OS. It's not that fast (although the 1.42 is comparable in speed with a recent vintage Powerbook, except for the video). But it will run productivity apps, email, web browsing, all the usual suspects. It would make a nice little media server. But it appeals to a completely different audience. OS junkies will dig it, but hardware junkies will look for something faster.
Personally, I think the Mini is cute, but I have no use for it. I do have a homebuilt HTPC in an Aria case which fits my HTPC needs. The Mac Mini just can't compare, but then again, it was never intended to.
So, if you want a machine to play around with, maybe learn some Unix and OS X. It's a great box for hacking (for good purposes, of course). But don't confuse it with a real computer, either PC or Mac.