I would say not to get one.
Ok, I just checked the only prebuilt mac clone I know of (as opposed to building your own). It starts at $400, then $155 to add Leopard into the mix instead of free or different operating system. Suddenly we are close to a mac mini in price. Ok, but I don't think OS X works with pentium processors, even dual core (might be wrong though.) If thats the case, we have to up the processor, which is $60. I guess we're now at $215 on top of $400, so, $615. Mac Minis run $600 new, or $500 Apple refurbished.
It DOES have a separate graphics card (as opposed to integrated, like the mini), and a bigger hard drive. It doesn't have firewire ports until you specifically add them in. Not a big deal for me, but for some it might. It also has another gig of RAM that the mini doesn't come preconfigured with.
However, the biggest thing for me is the fact that is difficult updates. Some that isn't a huge deal, some it is.
Everyone likes their own thing. I simply would prefer Apple supported hardware and software. Apple hardware also generally lasts longer in my experience (had a few logic boards go out, and a few LCD problems on ibooks). This isn't always the case, just what I have seen.
Over all, I think they're close enough in price and specs to not be worth the issues that different hardware would bring. A dedicated graphics card in the mini would be nice, but everything else isn't a huge deal for me. Or if it is, its something I can work out another solution for (external hard drive, adding my own RAM or getting it installed if I have to).