Windows 7 is a wonderful OS. It is certainly better than Windows XP, and it is better than Vista. Comparing Mac OS X and Windows is note particularly effective since Mac OS X is only legally available on their own (Apple's) hardware. Windows 7 has newer versions of DirectX, including the DirectX Compute Shader (to compliment OpenCL) and _does_ work with OpenGL for hardware vendors who include support in their drivers. Windows still is the majority OS of choice for software of any type. There are some wonderful packages that are Mac or Linux specific, but there are far, far more that are only Windows specific. Windows 7 can run as lean as Windows XP, and yet supports far newer technologies, and is far more usable. You can compare this to OS X 10.6 which will have a larger footprint, still support new technologies (yay for OpenCL) and yet cuts support even more for old technologies.
Really at the end of the day, it comes down to what do you need and want to do with your computer. If you hardware isn't Apple to start with, getting OS X to run on your machine is neither legal, nor is it easy. It is also prone to breaking with every new patch. If you run on Apple hardware, it is a very good OS. I have a mac Mini and a MacBook Pro (Dual booting MacOs X 10.5 and Windows 7 RC). Linux is a wonderful server OS, and has started to get better for desktop purposes. As a workstation piece it is very good, but still needs more software package support in areas. I run two Linux machines, a SlackWare server, and an Ubuntu development station. Still at the end of the day, Windows will be the OS of choice for me, and currently Windows 7 is the best of the crop. I currently own 5 windows machines, 1 server 2003, 1 server 2008, 1 Windows Vista Ultimate, 1 Windows XP (8 year old laptop), and 1 Windows 7 RC development machine.