Most people use Windows for two reasons, they don't have a choice because it came with the computer and because that is what most other people use. Neither is a a good reason to choose an OS.
Like twd I use Linux and have for many years. My computer came with XP MCE and I never see the need to boot into Windows and when I do I regret it. It is often months in between Windows boots. When I do boot into Windows I cannot use my machine for a good ten minutes. Norton takes over my machine for about five minutes. It wants to verify my license. Then it wants to update. Once that is done it wants to reboot because it has updated not just the signatures, but the program as well. The same thing happens with my Norton Firewall. Then Windows updates itself and wants to re-boot and nags me every ten minutes or so until I finally give in and re-boot.
Each time it re-boots it goes through a similar thing, albeit shorter. By this time I have wasted 10 minutes and forgotten why I wanted to use Windows in the first place. Lots of other software wants to update itself as well because I have not used Windows for months.
Finally when I do use Windows I find it to be an unrewarding process. It runs slower. It gives me a cluttered workspace because it does not have virtual desktops. Every email and download is slowed by virus and trojan checking software. The menu is messed up each time I install a new program because each program insists on putting it in its own directory in my menu and it wants to clutter up my desktop with useless icons.
In Linux, the same computer boots in half the time. It pauses to ask me for my user name and password and then it goes right into my desktop. I do not need an antivirus program, malware detectors, or trojan removers. My system tray is relatively uncluttered. I can get to work right away. When it wants to update I get an icon in my system tray that tells me I have updates. If I choose to update, it asks me for my password and updates as I have directed it to without prompting me to re-boot. Even if the graphics card driver or window manager is updated I can keep on working. If I want to take advantage of the new drivers, I can log out and restart my window manager. I am never nagged to re-boot. I can go months without re-booting my system. Some people even go years without re-booting.
In Linux, I have no virus worries, no trojan worries, no bot worries, and no malware worries. Nothing installs without my giving it permission to. My home page is not hijacked by active x scripts or other Microsoft specific headaches. In short my system is secure.
I also never need to clean off my desktop from useless icons unless I am running a Windows program inside of Linux which still manages to put desktop icons on my computer. When I install something it puts itself inside of an existing sub-directory all neatly organized by the desktop manager.
When I am multi-tasking which is most of the time, I have multiple desktops to spread my work around. I can have more desktops than I can keep track of. I can switch between the desktops in several ways. I can use an icon on the taskbar which even shows a picture representation of the program I am using on that desktop. I can use my mouse buttons and wheel, I can press a hot-key or I can move my mouse cursor to the top right hand corner to see all of the desktops I am using on one screen.
Not only is my system faster and more feature filled, but it is better looking than any version of Windows thanks to Compiz-Fusion which gives me a 3D desktop experience with plenty of eye candy. I have lots of choice of desktop widgets. My system is infinitely configurable. Unlike Windows where I am locked into what Microsoft says my experience should look like, I can take control and change it in very fundamental ways. I can change the window manger. I can change the desktop manager. I can change the kernel. I can add new features to any of the above. In addition I can change the themes just like any Windows user can regardless of the choices I made in desktop manager, etc.
Linux isn't for everyone. If you play lots of Windows games then, you will need at least a dual boot machine. If you only need compatibility with Windows then you are probably okay with switching completely to Linux. There is very little that you cannot do in Linux that you can do in Windows. Many Windows programs have open source equivalents and they usually are available for free.
Windows users do not need to fear using Linux. A Linux machine works much like a Windows machine. The differences run deep, but most are invisible to the eye.
My computer came with Windows XP MCE and I paid for it, hidden in the cost of the machine. I was never given the choice to buy the computer without an OS at a reduced price. Instead I paid for something which is useless to me and that is wrong. Users are being held hostage by Microsoft and OEMs. This is a monopolistic practice that has made Microsoft rich enough that they can threaten small developers and big OEMs alike into doing it their way.
Using Linux is my way of fighting back. Linux is the only thing that Microsoft fears. The reason is that Linux is not a company that they can buy out or push around. It is millions of small users and developers that have bought into an idea which is to take back control of their computers. This is the last thing that Microsoft wants.
When they developed Vista they were even more high handed. They built digital rights management right into the computer. They went beyond the usual. Big Brother is not just watching your windows license, but MS is watching everything that you do on your computer, every song, video or program must pass Microsoft's test or you can find yourself unable to use your computer and more often than not legitimate users are being locked out by high handed tactics and failure prone software that MS produces. In the license for Vista MS has the right to lock you out from using your computer, the one that you bought with your own money. This is MS's vision of the future.
Things do not have to done this way. You can buy a Mac and use OS/X which is a premium computer make no mistake about it. I have never heard a Mac user complain about their hardware or software. They are very loyal to their brand. That comes at a premium price as was already mentioned here. The other alternative is to take back control of your computer by switching to another OS. Linux is only one choice and there are hundreds of choices of Linux. You can run BSD which is what Mac OS/X is based on. Linux, OS/X and BSD are all branches in the Unix tree.
Unix can be confusing until you get used to the commands and the structure. This is where graphical interfaces come in. You can safely use Linux without ever using the command line. Your Linux experience can be on par with the Windows or OS/X user.
I am obviously happy with Linux. I have total control of my computer. I am more productive and I know that my computer will keep up with my needs for much longer than if I ran Windows. People can run Linux on much older machines than mine. It can run from a mini CD (50 MB) or less. It can run from a RAM disk, a usb key, CD or even a floppy. Some versions of Linux such as Sabayon are much bigger. A full installation tanks out at about 14 GB. But even this is small compared to Windows. Try running Windows from a 15 GB hard drive with the OS plus all of the software you are ever going to need. It will probably run quite slowly. Instead 15 GB in Linux gives you a sleek bleeding edge OS with a 3D graphical desktop.
You should consider Linux if, you don't play a lot of games, you do not have licenses for much of your existing software, you are on a budget or would like to save money, you want the fastest and latest experience, you find Windows too high maintenance, and you like trying new things. If you are easily frustrated or are not very computer savvy then you might want someone to step you through the process.
I know more about Windows that I do Linux. As a Windows user I tinkered with my system and got to know every possible problem and learned through the school of hard knocks. Since starting Linux, 7 or 8 years ago, I have never had a hardware or software failure. I have seen the desktop manager freeze once or twice, but I have never had my system go down. I have never lost data or had to start from scratch. I still have my original bookmarks from when I first bought this computer despite installing dozens of different versions of Linux on the same hard drive. My home directory has never changed in all of that time and I have never had to back it up to any other media.
Linux is free, but it not second class in any way to Windows. Using a PC does not = using Windows.