OSX can be installed on a non-Apple computer, although it’s not quite a simple as just popping in an installation CD. You can see how to do it here.
Personally, i think that OSX is an excellent operating system, but i really don’t think it’s better in many substantial ways for the end user than Windows 7. Both operating systems have their own ways of doing things and their own little quirks and idiosyncrasies, and i think that preferences for one or the other tend mainly to reflect which one you are more accustomed to. I prefer Windows in many ways, and if someone handed me a honking powerful Mac Pro, the first thing i’d probably do is install Windows 7 on it.
Apple’s big market is really in hardware, and more specifically in laptops, iPods, iPads, and iPhones. Those are the things that have driven Apple’s incredible success over the last decade, along with stores like iTunes. Mac desktops constitute a tiny percentage of the market.
You’re right at some level that Apple desktops (i’m talking mainly about the Mac Pro and not the iMac here) use components similar to many other desktop computers. For the most part, a hard drive is a hard drive, and you can buy hard drives and motherboards and memory of the same quality that Apple uses simply by going to an online retailer like NewEgg.
But one area where Apple seems to do better than a lot of other companies is in putting their hardware together, and making sure it works. The construction of their laptop chassis is also very good quality. Also, if something does go wrong with your system, my reading suggests that Apple is often much better with their customer service than other computer retailers like Dell, HP, Gateway, etc.
One of the greatest things about Mac desktops like the Mac Pro is the design of the case itself. If you’re the sort of person who wants to be able to add his own hard drive, the Mac case makes stuff like this incredibly simple and neat. Look at how well-organized it is. Yes, you can buy third-party computer cases that are just as good, but this isn’t the sort of case you’ll find on a typical turnkey desktop computer.
Actually, for this reason, if i bought a Mac Pro, i would order it with only a single hard drive. Because Apple is a hardware retailer, they tend to shaft you on the price of add-ons. If you want a 1TB second hard drive in your Mac Pro, it will cost you $150; if you want a 2 TB drive, it will be $300. I can go on NewEgg right now and grab 1 TB drives from good manufacturers for well under $100, and 2TBdrives for not much more. Combine this with the ridiculous simplicity of adding a hard drive to Apple’s fabulous case, and you’d be crazy to pay the price that Apple charges for extra drives.