I am a hardcore Linux user, and I have tried out the majority of the distros out there. I used to use red hat back when it was free.
My favourite, however, is Ubuntu, simply because if you EVER have a problem with something, you can search it on google or their forums and find exactly what you are looking for. It's usually quite a bit tougher to find help on other distributions.
Also, standardization is good for anybody, and since I develop a lot of things in my spare time, developing for Ubuntu as a whole is a good idea, because it will then be more compatible. I mean it's linux, it will be compatible with almost everything, however sometimes it makes a difference.
That all being said, I also use Windows quite a bit, so I dual-boot all of my computers. Currently it's Ubuntu with Windows XP, however I'm going to take a look at windows 7.
Also, check it out - Ubuntu 9.10 is being released on the 29th and Windows 7 is being released on the 22nd; I wonder how this is going to work out :biggrin: