For my daily work, I am booted into Ubuntu Linux, always whatever the current dev version is. And it is tremendously comfortable. However, I am not religous about it, I insist on using the best tools for the job, and I don't mind a bit of effort occasionally to help improve free software to make sure it is even better than the proprietary counterparts. There is a short list of applications which I find myself forced to use OS X for though, even though I get annoyed at using OS X:
I can sorta get by with using Open Office Impress instead of keynote, but you know what? I don't want to get by. I make presentations for one reason: to change the way people think about something. I want those presentations to be as smooth and polished as possible, and I want the tools I use for those presentations to help me get my opinions onto the screen easily, and to help me as a presenter remember the outline that I'm working from during the presentation (I usually just show pictures or one or two words on the slides). Keynote even has a "publish to Youtube" button!
I'm a terrible artist, I have very little ability. Yet I often use a whiteboard or notepad when explaining complex technical things. OmniGraffle helps talentless old me get some pictures down which explain the concepts but don't hurt my eyes. I think Inkscape is poised to be a better alternative to OmniGraffle, but I really need a library of basic shapes to get my diagrams jump-started. I think something like this may be in the works but I haven't figured out how to do it yet.
I've tried using Kino a couple of times, but it just doesn't compare to the amazing editing UI of iMovie, and there is no linux app that I have found that lets me create DVDs with menus anywhere as easily as iDVD. I love free software, but when I'm working to a production deadline (I do video work for a group I volunteer with) I just need to slap some menus down and get the DVD burned before the projectors come on, and I can't (yet) do that with any of the linux video applications. PiTiVi is a lot easier to use than Kino, but you can't even save a timeline yet, which is a bit of a blocker for any serious work.
And screenflow, wow. The most amazing application I have seen in a LONG time. We could totally do something like this in Ubuntu, this is the gold standard for screencast software.
I hope that in the coming months I'll be able to find suitable Ubuntu based replacements for these applications. I know it's possible, and I'm willing to try and help with open source projects that have a clear vision of beating these proprietary tools.