Which OS do you use, and also, which do you prefer? Here's my thoughts...
Mac OSX makes a great OS that is stable and easy to use. However, good luck if you need tech support and oull probably have to get a mortgage on the house in order to ever get one. Their price point puts it outside the scope of most people. Also, for techies like us, it has the added disadvantage of not being able to mess around with the insides.
Windows makes a fairly decent OS (as long as you leave Vista out of the equation) with XP and 7. It's pretty stable and reliable plus it's easier to get software for than Mac or Linux. One thing to look out for with Windows is that if you want to change out your hardware, due to OEM restrictions, you'll have to reactivate Windows if it will let you and it may not even do that. So, if your mainboard fries and you need a new one but can't get ahold of an original, you may be out of luck. Although, I have done this before and gotten it to work. That was with XP and I'm unsure with 7 (and won't give Vista the time of day). Viruses are another hassle here and with all the software you need like antivirus, anti-malware, registry mechanic, etc. to keep it running smoothly...these are all things that work against Windows in a BIG way.
Linux comes in many flavors and not all are equal. I use Ubuntu 10.1 almost exclusively (although I have Windows 7 on my laptop). I'm in Ubuntu right now as I type this. When I swapped the motherboard out one time, it didn't freak or tell me I'm in violation of the OEM agreement. It didn't just stop working. What it did was say, "Oh...I have new hardware. Well, I'd better download and install the drivers." And...it did it! Easy as that! I think I downloaded an antivirus for this but honestly, I seldom use it. I run an extremely stable platform that gives e everything I want...power, speed, ease, reliability, customizability, and geekiness! 
Oh, and did I mention it's free? When I had to install Netbeans for a class I took at University of Phoenix, the teacher had asked us during a discussion question how we did it and if we ran into troubles. I said, "I clicked on Appliciations -> Ubuntu Software Center, searched for Netbeans, clicked install, put my password in and pushed enter. Then I went and made coffee while it downloaded and installed Netbeans for me automatically, storing it in a place where I'll know where to look (Applications -> Programming)"
So, to answer the question as to what OS I use and prefer, I have to say Ubuntu 10.1 all te way. And you should see my boot up and shutdown times. My OS will shut down in 10 seconds or less unlike Windows. From the time I push the power button, I'm less than one minute away from my system being usable. Of course, my system will stay running for months on end without need for reboot so that hardly makes any difference!