Also my first post. This could be difficult to answer as "completely switched" could be relative. I have personally been using the Linux kernel with various distros since 1997. I started with Slackware, which I still use today. I also have Ubuntu on my Laptop - which runs great!
Recently (last week), my girlfriend had a HDD crash in her HP DC7900. She asked me if I thought it would be a good investment to use this opportunity to buy Windows 7 - "as it would be a more effective OS going forward" -she commented.
I convinced her to let me install an even more effective OS, Ubuntu 11.10. Thanks Canonical for an absolutely great OS! Even better for her transition, I found a theme on gnome-look.org that (with a little bit of work) makes it look just like windows 7.
Now if I could just get the cat to switch, the whole family would be using using Linux.