MacOS X user
I've been (and I still am) a free/libre software advocate for many, many years. I've also been an active member of the community since 10 years ago, when I contributed my first translations of technical documentation to the Lucas/Insflug project. With this background, my friends are a bit confused because I've recently become a MacOS user (I'm a bit surprised too). When I bought my MacBook, I decided to give an opportunity to MacOS Leopard. Even if MacOS has a UNIX core, I was afraid of feeling strange, so I also bought VMware and I installed my beloved Debian in a virtual machine. My main fears were: a) to become less productive (I'm a Linux power user, so I'm quite productive using UNIX tools), b) to miss some critical apps, c) to loose control over my own computer.
Regarding the first one, I've discovered that I can be very productive with the Mac. There are a number of amazing applications for the Mac that are designed to boost your productivity. To my amazement, there are even some "semantic desktop" applications that really work. I'll try to cover some of them in future posts. Anyway, for complex tasks, I still have the ability to fire a UNIX console with bash.
With respect to the applications I use everyday on Debian (all of them open source, of couse), fortunately many of them also exist for the Mac: Firefox, Eclipse, Emacs, Apache, Python... Other ones, particularly OpenOffice, are a bit behind, although this is expected to change in the next months. I think the application I miss the most is Evolution (the mail client of the GNOME desktop). Evolution is, by far, the most complete mail client for the desktop, built by people who really use e-mail for their everyday work. Note that I'm not claiming that Evolution is the perfect and definitive e-mail client, it still has its flaws (particularly, stability), but I have no doubt on its vast superiority over the alternatives (side note: I still can't believe that there are happy users of Outlook). I've tried three alternatives: Apple Mail, Thunderbird and web-clients (GMail). They're OK, but they cannot make me forget Evolution.
Finally, with respect to my latter fear, the situation is a bit worse. I've effectively lost the complete control of my computer. It is executing code that I cannot see or compile by myself. However, from a pragmatical point of view, the situation is not really different than in other devices (my mobile phone, my Palm, my photo camera, my video game console, my iPod, my TV set, my car, and of course the firmware in many devices inside and outside my PC, etc... all run proprietary code). Unless you are rms, you cannot claim that you are not using a single line of closed code. Please do not misunderstand my point, but you have to draw the line somewhere. I recognize the value of running free software as much as it is possible, as demonstrated by the fact that I use much more free software than the vast majority of the computer users. By switching to a proprietary OS, I've moved my personal line a bit backwards, seduced by the charm of Mac. But I don't feel this is a betrayal to my principles, which I still hold. I've always defined myself as a free software advocate, not as a free software partisan.
I'm not trying to convince anyone or to defend my point. Besides my laptop computer, I'm still a very happy Debian user on my personal desktop PC at home, and at the PC at my office. Everyday, I spend more hours using Debian than Mac. Debian is installed in every computer I own. In my eyes, GNU/Linux is the best platform for almost every task, and in particular, for software development and internet, which are my main occupations.