My grandma used linux for 3 years before buying a new computer with vista on it. It did everything she ever needed it to do. Unlike the OS it replaced, windows 98, because of me having to drive out constantly to fix viruses (I'm too cheap to buy an OEM copy of XP) it never crashed, she never called me for support. Ok, that is a lie, she called me 3 times. Once to install the dvd codec (I guess I forgot to install that in my setup), and once to ask me why a website she liked would not work (it required java, which I then installed). After that she was happy. She actually likes vista too, but she comments on how hard it is to find software that does what she wants and how her 'old' pc did everything she needed. Of course this is because I knew exactly what software she needed to do what she does on her computer. I delivered it in a working state.
I use a mac, why because it is just as secure as linux imho, is unix (which is a requirement for anything I use), has a nice user interface and consistent feel, can play blizzard games (only pc games I care about anymore) , great looking stable hardware, and has all the software I require.
I have never bought software from a store after I left windows xp. Even with my mac, with the exception of a few blizzard games (which I had already bought for windows) and textmate, the rest of my software either came from apple or is open source. My family has also learned about open source and now calls me before even thinking about looking for software to ask me if there is a 'free' software that does the same thing. They all now run open office, vlc, firefox, thunderbird, you know the drill.
Usability is all in well how you use it, obviously if gaming is your number one concern, it makes sense to own a 360 or a windows pc. If a powerful text shell is a requirement for you, then having a *nix environment is a must.
The requirements listed in this post were secure, easy to use, and gets the job done. That's linux or unix (mac) in a nut shell. It might not be easy to you guys who have never used it, or are trying to use your windows ideals in it, but for us who grew up with it, it is way easier then using windows. My requirements for a computer are a good web browser, a powerful text shell and tools to script and automate tasks, good solid web development tools for php, python, and ruby, a nice music player, a nice video player, multiple desktops, a very powerful gui text editor (goes with solid development tools), a instant messaging client with support for google talk, msn, and icq, a powerful command line text editor, ssh client, openvpn client, music recording software, ability to launch X11 applications via ssh, truecrypt, and because a lot of clients feel the needed to send office documents; a program that can view MS office documents.
Out of the box, I can get this setup with linux or mac. In fact mac comes with most of the software I need out of the box. Windows is a bit tougher because I have to install linux on top of it (essentially) via cygwin to get a good shell and run the X11 apps I need for my job (yes, some of the software we use can only be managed via a X11 application and it is easier to do it remotely then walk to the server room). And really there is no OS out there that can't meet those needs, the question is how much work to get it together and what interface you are most comfortable with.