Mac on Linux, is it legal?

It's against the EULA, but in general, each provision of a EULA is treated separately (at least, according to Wikipedia), i.e. so a part of a EULA might be found to be legally non-binding. The legality depends on where you are, but if you're in the US, I would guess it's illegal because of the DMCA. Apple sued a clone manufacturer(Psystar) several years ago for selling machines with OS X installed on them. They sued under several different legal reasons, but one of them was that Apple uses encryption methods protected under the DMCA to keep OS X from being installed on non-Apple hardware. The court found that indeed the DMCA was violated and Apple won.

It is definitely a EULA violation. Violating a license agreement is like violating a contract. It gives Apple the right to sue you. Will they? Probably not. As long as you aren't getting in the way of their business. So is it illegal? I would say yes. Is it criminal? No. It's simply a civil contract violation. You have to make up your own mind. Personally, I suggest just getting some Apple hardware because it's great.

Legal or not is a grey area for OSX on non-Apple hardware. I think, for personal use, the most that will happen is that they won't give you tech support