Not sure if "usable" is really the right word, but for me it's just so much more of a pain to work with the audio and video tools I found on Linux that were supposedly "best of breed" (Rosegarden and Ardour for audio, Kino and PiTiVi for video. I tried to get the Cinelerra and the highly-rated OpenShot video editor to work, but ran into lots of issues.) The audio tools were very difficult to set up, and the interfaces are nowhere near as slick as Mackie Tracktion 2 for Windows. Kino and PiTiVi were a total joke.
I initially bought into the idea that since it was open source software, I could fix and improve the software I wanted to work with. But the IDEs that I tried are nowhere near the level of VisualStudio, the languages they were written in I was unfamiliar with, and I just don't have the time to learn right now. Never mind that coding for audio and video applications is difficult to begin with. I might get into this someday, but I didn't want to deal with it now (I usually LOVE this type of stuff).
I did have great successes with converting two of my computer virus-prone relatives to Linux. One works for a major financial institution that has web software that won't work without the Microsoft JVM! Also, Windows security updates wreak havoc on the software. My solution was to install Ubuntu, and use Sun's VirtualBox with a working snapshot of XP. If he ever got a virus, he could just revert back to the snapshot. The financial software now works perfectly, and for the first time in years, he actually like using his computer. I also put my dad on Linux Mint, and he keeps telling me how much he loves it (aside from a few minor annoyances, but nothing like dealing with viruses and malware on Windows).
To be fair, both were using Windows XP - another virus-prone relative who I do tech support for has been on Windows 7 for a few months now (protected primarily with Microsoft Security Essentials), and has not had an issue yet, and loves her computer now as well.
There were initial hardware compatibility and other issues with both of these installs that took a bit of time for me to work through, but both systems are rock-solid stable now. I think Linux can work very well if you are just primarily browsing the web and just need to work with Office-type software (Sun's OpenOffice works very well if you're not doing anything crazy). It can also work well in other situations, depending on your exact needs.
Having switched back to XP myself, I find myself missing a few of the nice features on Linux now. The Bash shell is wicked powerful and a quick way to do system administration, having multiple virtual desktops is great, and I love how configurable pretty much everything is. Also, not having to worry about viruses nearly as much is awesome. I'm sure I will try it again someday.