Before the first, sad, day that I ever saw the Windoze operating system, I was a happy user of IBM's AIX. Then came Linux, which I have been using almost exclusively for some five years.
As a developer, I like to have the occasional play on alternative platforms, so I have lately acquired an OEM Sun SPARC machine. I have set this up with Solaris 9. (Solaris 10 won't run on this older hardware.)
My opinion? Cool. System administration tasks are as easy as they should be and there is a wealth of good information out there, to be had for the Googling. As I can't work happily without my GNU software, I have downloaded a load from Sun Freeware – a most excellent resource.
My Sun machine is now doing “hot” backups of my main Linux box.