There are many OS out there, some new ones like QNX are looking promising. How ever it is not for the general public use. That is, you can't just put the OS cd in the cd-rom and expect it to work properly without any intervention.
The strength in the MS OSes is the large market audiance and the abundance of software. At the moment no adobe software runs on any other platforms apart from Windows and Mac without any emulations. For graphic designer this is a very important factor.
I've tried almost every forms and variants of OSes, and even have written a real-time OS. But what it comes down to is what you need it for. If you want simplicity go with Mac or windows. For greater stability go with some variant of unix/linux. Or lastly, just build your own and see how complicated it is. But with all OSes there are disadvantages and advantages.