Four points:
1) Looking at their site more closely, it is clear this is based on the OSx86 project. Basically, buying this computer is buying a PC that has hardware that is guaranteed to work with Leopard, and I guess has the EFI emulation in place. You could do the same on your own but I'd assume it'd be pretty technical and you'd have to be certain to get compatible hardware. Put another way, this is not really a Mac clone - think of it as an OSx86-ready package. Given this, I'd shy away from it if you aren't technically inclined. Certain updates can break it, so you have to follow the "Hackintosh" community to know what's safe and so on and so forth.
2) I think they get around the EULA because they are selling you a retail copy of Leopard. Thus the customer is the end-user. So, basically, YOU buy Leopard, and then "ask" them to install it for you. Thus, technically speaking, YOU are the one violating the EULA, not them. I may be wrong of course, but I think Apple may have a difficult time stopping this.
3) This fills a need that is sorely lacking, namely for gamers. Gamers don't want or need a Mac Pro, they just want a video card in something thats not an annoying all-in-one, but Apple refuses to provide it. Gamers also tend to be technically inclined. Thus, I can see this doing well with the small subset of gamers that would like to try a Mac.
4) My biggest worry would be "future proofing" - sure this works now, but you are basically relying on an online community to ensure compatibility with future updates and future OSes. And you can bet Apple will be working hard to make that as hard as possible. If the Linux community is any guide, Apple won't be successful,but its likely what's available for this machine as part of OSx86 will lag considerably behind what is official.