I started using Linux 3 years ago after my windows crashed totally. I had briefly tried a Slax Linux live CD a couple of years before that when one of my computer geek friends (an Iranian) showed it to me and tried to convert me from being a windows user. He himself was/is a system administrator and a Slackware fanatic. At that time I didn't understand all he said about freedom, open source, full control over your system etc.
When the day finally came when XP took over 10 minutes to boot, full of trojans, pop ups and what not I remembered Linux. I managed to get online and read about Ubuntu and decided time was right to give it a try instead of a fresh install of XP. I downloaded Ubuntu 7.10, burned it and have not looked back since. I was right at home!
Ubuntu has come a long way even from 7.10. I would say that it is pretty much as user friendly as windows or Mac OS, at least this version (10.04). Light years behind Microsoft when it comes to intuitiveness? No way. I'd say it is ahead, specially for most desktop user who just want to surf, chat, social network, download and watch movies, burn CD/DVD's and things like that. It is all so simple, elegant, fast, secure. Geeks on the other hand can go further and do what ever they want with the system, tweak it, change it and the community is the best if you ever get stuck.
The philosophy, security and the community behind Linux and Ubuntu should be enough to switch IMO, unless you are a gamer or use Adobe in your work. I still have a dual boot for that purpose and hope to have a linux version of Adobe production suite!