I wanted to be a Linux user since I first heard about it, and over the years I've made several attempts failed attempts to switch with various distros, Redhat and Mandrake mainly. All of these started the same way, I'd create a dual boot, I'd set Linux as the default with all good intentions of only booting Windows when needed. But these attempts always ended with me rushing to select Windows on every boot, finding using Linux day to day just too hard.
I did finally make the switch and have been using Linux as my primary OS at work and home for several years now and can't imagine switching back. Thinking back there are a several things that changed from those early attempts, few of which are directly attributable to improvements in Linux. The trigger oddly was Vista, or more specifically Peter Gutmann's analysis of the cost of the Content Protection in Vista. This set me to worrying and raised questions about whether I wanted to support what I still consider to be a destructive course. So I resolved to make a real attempt to switch, sure I'd still be paying for OEM copies of Windows with every machine, but at least I wouldn't be contributing to the market share. I also hit the disturbing revelation that I couldn't be sure my PC was secure anymore, there was just too much malware out there.
At around the same time a couple of other factors came into play. My work started experimenting with Linux desktops and so lucky me, I got to have SUSE Linux Enterprise Desktop on my work machine. Personally I didn't find SLED very good, but these experiments opened the door. It became acceptable to have a non-widows install. You had to support it yourself, IT only supported the Windows installs, but it was acceptable.
The only directly Linux cause is that Ubuntu Dapper Drake was starting to get a bit of buzz in the tech news. A Linux distribution targeting ease of use for normal people. I love the power of the Linux command-line, but I really don't want to stuff about keeping my machine functional, particularly with a work box. So Ubuntu with it's easy administration was ideal for my purposes. So I ended up with Ubuntu installs at home & work.
The final factor that allowed me to successfully switch as the emergence of Firefox. Or more generally a growing acceptance that the web had to work in more than just IE. With the web-standards movement and the emerging notion of the web as a platform more stuff just worked. So many of the thick client applications I used to need were replaced with web applications. That and there were perfectly good open source alternatives for the rest. GIMP and Open Office are different, but they work perfectly well.
My usage has also changed, back when I first wanted to use Linux I used my PC heavily for gaming, these days I use it for Email, Browsing and Coding. Activities that are much easier to do cross platform. So indirectly the growth of gaming consoles has made it easier to switch.
So these days I couldn't imagine switching back. I still have a Windows install for the occasional game, but that's all it's used for. I don't even have it connected to the network at the moment which makes it far easier to secure. Finding software for Linux is easy, and while sometimes it doesn't work the way I expect, it's never malicious. Last time I tried to find some Windows shareware it took me days to find something that didn't bundle spyware. I've transitioned so completely that the UI changes in Windows 7 confuse and infuriate me. I believe the UI changes are probably good, but I have yet to get used to them since I use it so infrequently.
So I personally wouldn't switch back, but is Linux ready for the Desktop generally? Ready for non-technical users? Almost. After upgrading to Natty I turned Desktop Cube back on and all my title bars disappeared. I eventually figured out that every Compiz setting had been turned off. So when I switched it back on to get desktop cube the title bars, window resizing etc were all disabled.
This wasn't a big deal for me, took me all of half an hour's googling to work out what went wrong. But I think of my parents hitting the same issue and it would have been a much bigger deal. For Linux to really take off on the desktop these kind of issues need to go away. It needs to just work. In fairness it does now 95% of the time. But that extra 5% could put a non-techie off for life.
Then again, when I ran Windows as my main OS, I had to re-install every 6 months to keep it running. So perhaps it isn't such an unacceptable issue after all.