Why bother ?
I really tried hard to love Macs ; since the first time I laid my hands on the square mice of an original beige Mac 64 (Kb, that is) until now, maybe once a year I manage to get a couple of days alone with one, waiting for the magic to happen, and I leave desperate. I have all reasons to love Macs. I even run NeXTStep 3.3 on a sparcstation 20 for the fun ! I wish EFI becomes more widespread, and I taught myself the basics of objective-C and Xcode (was called AppBuilder in the NeXT realm, I know it vastly improved since). But nope. Doesn't work for me.
Firstly, the much vaunted Mac hardware is (to put it mildly) a filthy pile of standard PC components glued together in the worst possible manner, with barely enough forethought to boot MacOS. As most people will only run MacOS, it's all right, but you won't sell me the system at a premium based on the engineering alone. I don't include the case outside design in my concerns, I admit it's fashioned, clean and desirable, but there should be a computer inside first thing !
Secondly, price. It goes with Apple like Leica : all things being equal, the apple kit is not so much overpriced if you were to build a similar PC kit. But you wouldn't ! Why shell out such an ungodly amount of money on something that won't last more than a couple of years (and be considered underpowered in 6 months) ? There are a few people that really, really, need to pay the premium because their job dictate it, and they are able to recoup their investment quickly enough. I very much doubt it's the case of the majority.
Thirdly, software. MacOS isn't windows, neither is it Linux nor BSD. This may be OK to users, but it gives me (personally, I make no general consideration here) claustrophobic moments. I won't speak of windows for I don't use it being nearly linux only since 1998. To me, it's not only that it's simply alien (I manage quickly enough the shortcuts not to bother too much about the GUI specificities), it's outright oppressive. For the fun of it, I tried linux on various Macs, but the hardware is so restrictive too that I'm forced to conclude that a well trimmed PC is better in this role.
Conclusion : if I could turn my PC into a Mac for those moments I need one, I'd happily do it instead of buying the full kit, because I know MacOS is unlikely to become my main system. Maybe I could spend a bit more time with it (say about 25% of what I do), but the price has to reflect this secondary nature. At the moment, a hard solution like EFI-X seems perfect, but still overpriced. They sell you what amounts to a bios + hard drive at a full computer price.
End note : BSD is no more Unix, nor Posix than Linux. I very much doubt there's any original Unix code left in BSD, and the tenuous link between {Free|Net|Open}BSD and the original port of Unix at Berkeley by Ken Thompson is in effect a tale of lore intended to make BSD users feel more aristocratic than the linux commoners crowd. Not that it didn't happen, but there's no magic in computing, and I very much prefer a clean rewrite of a function rather than being bitten by a buffer overflow left in ancient code because it has been written by a semi-god no one dared to contradict ; and luckily, this isn't how it's done, BSD code has much improved over time. But at the moment, hardware support is better in Linux than in all BSD around ; linux even runs on some platforms NetBSD doesn't (and vice-versa). When it comes to various cards addons, the difference is evident. To ice the cake, for a long time, linux has been the only system SMP compatible, so as soon as you had to animate more than a CPU (or a core), there was no choice at all. Free and Net BSD are slowly coming to par in this area, but I'll wait a bit more bug testing before considering a switch.
About Posix, well, Linux is fully Posix with GNU extensions, like BSD is Posix with BSD extensions. That's all there is to it. Hey, even Windows NT had a Posix layer you could bolt on ! In the beginnings, around linux 1.2, posix compatibility was admittedly a guesswork because nobody had the means to offer Linus Thorvald a full Posix specification. That's 10 years old news.