I started using the Macintosh OS almost 25 years ago and still use it to this day.
I appreciate the fact that I have never, ever installed an anti-virus program and have never had an infection of any sort on my machines.
I use Windows machines at work, so of course I have experience working on both environments.
Both the Mac OS and Windows have had their ups and downs through the years. Some times they have been slow, or unresponsive, or crashed at innoportune times.
But they have also improved drastically in each succeeding generation.
The majority of people I know have stuck with XP, saying that Vista is still too buggy. I am sure MS will iron out the bugs eventually and it will be a great operating system.
My little brother uses Linux (don't know what version) most of the time. I am not familiar enough with it to express an opinion about it.