Yes, you CAN use the upgrade version for a new install. You're not supposed to. Amazon sells both the upgrade and full version (called "The Box Set").
Some people like Mac OS. Personally, I don't care for it, but I'm not an OS bigot. I don't think it's "better" than Windows 7, a lot depends on what you want to do with it. I perhaps have an unusual perspective because I'm a developer of commercial software that runs on Windows, Linux and Mac OS, and I get to see the warts underneath the surface of each of these operating systems and how each provider deals with changes between versions. Apple is by far the worst and hardest to deal with in this regard, breaking interfaces with nearly every single update, and, unlike Linux which does much the same, Apple won't tell you what changed nor is it easy to find out. But this is likely of no interest to the average user (until they find that installing an OS update breaks their favorite application.)
Mac OS is popular here because there are a lot of Mac fans who would like their familiar environment in a cheaper and more portable package, and the Mini 9 was one of the first netbooks to be "hackintoshed".