Well, it's only been a short while, and I have already seen two of my friends switch a friend each to Kubuntu.
It seems the concept of "pay it forward" may actually work in the real world. I guess time will tell.
You may also find this post on the Ubuntu Forums to be an interesting read on what people find is their last hang-up on switching completely to Ubuntu, or what finally tipped the scales for them.
For some, it's a working Adobe PhotoShop via Wine. For others it's the availability of a comparable Office Package. Whatever their reasons, people are continuing to make the switch daily.