Price: It might not be for you, but free-as-in-beer is a big selling point for me. I can get a fresh new version every year or so and never have to pay a dime to upgrade.
Security: It has a long tradition of being extremely secure, especially when paired with application software that make it easy to use the built-in security tools. Case in point Uncomplicated Firewall as a frontend for iptables. Both systems have major bugs and security flaws from time to time. As a major generalization such bugs in Linux (and associated other programs) get fixed quickly; hours or days. Those in Windows take days or weeks, depending on the position in the patch cycle.
Convenience: I can take a working disk image and roll it out to an infinite number of other machines, all of which don't necessarily have to be identical. This means that I don't have to spend even a second thinking about licensing and getting sued by the BSA. When I want to install a piece of software, VLC for instance. I don't have to open up a web browser, search Google for the author's web site, click the download link, wait, click the executable, click through the installer, and finally click on the application icon. I open up a window, type sudo apt-get install vlc and it does everything for me. In the mean time while that's working I am free to do other things. This is especially nice when I'm setting up a new system and I want to install three dozen programs. On Windows I'd spend an afternoon going to web sites, running the (non-signed) executables, and clicking through installers.
Power: I'm most productive when the distracting crap is swept away. Give me the simplicity and power of TeX over MS Word any day. Likewise for using vi and gcc rather than MS VS for my horrific excursions in C.
Applications: Your applications might not work on Linux, but the ones I use do, for the most part. I like Firefox and its many extensions. It's on Linux and it's fast. I like the Gimp because I don't have to pirate it just to fix redeye in photos (I'd love to see the number of home Photoshop users as compared to home license sales). It's on Linux and it's fast. I like to use Thunderbird when I want a native email client. It's on Linux and it's fast. I like to use TeX and LaTeX and LyX to compose documents. They're on Linux and they're fast. I like to use Pidgin to converse with folks around the world using the MSN, AIM, and Jabber protocols. It's on Linux and it's fast. I like to use VLC to watch movies. It's on Linux and it's fast. Did I mention that Linux is fast? Why pay £50-£200 when something that's free does everything I want?
Most people don't need very much software that doesn't have a free equivalent.
Flexible: I have an easy time setting up systems for use as kiosks or other single-purpose machines. For instance, Mythbuntu gets me a working, fully configured media capture and playback system in about a half hour. I have a jukebox system in my house that I use to play music when I have houseguests. People are incited to pick a few songs and create playlists. It all runs on Xubuntu, but my guests would never know that. All they see is AmaroK running in fullscreen mode with the DPI settings cranked up to 150. Since it uses a robust database (MySQL) for its indices, it is able to search my 50+ GiB music collection quickly, even though it's only running on a 500 MHz PIII with 192 MiB RAM. Linux loves networks and makes lots of somewhat interesting tasks easy. It would take just a few minutes to set up a system by which all my local documents are archived to a remote server a few times a day. I could have it automatically back up an image of my system disk the same way, should I ever need it for recovery. I can easily access my computer remotely when I'm away from home and I don't have to worry about its security. I can have it send me emails when there's a problem. All of this stuff doesn't require a special "premium" version. It works with stuff that's either already installed, or 15 seconds away. An open, well-documented system like Linux is vastly configurable in this sort of fashion, even if you're not an expert sysadmin.
Philosophy: Some people ascribe to the beliefs and ideals of the FSF and Free Software (free-as-in-freedom) in general. They believe that it is beneficial and that the free philosophy provides extra incentive to promote software freedom. They would say that there is a moral benefit to using Linux and other FOSS.
Community: Some people like to feel like they are a part of something bigger than themselves. They like to collaborate and work on things they deem important. Heck, why am I writing this? I'd doing it because I like Linux and I'd like other people to like it as well.
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What software do you need that isn't available for Linux? Maybe we could help you discover near-equivalents and/or alternative workflows.
The software that I need that has no good equivalents on Linux are:
SolidWorks (3D relational modeling)
AutoCAD (2D absolute modelling (yes, I know AutoCAD has a 3D mode, but it sucks)
Various DirectX games (though I don't game much any more)
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Linux certainly isn't superior in all aspects, but it suits my needs and provides me with something of a hobby. I can use it to both buckle down and get work done, and as a plaything.
