I have used OpenBSD since 3.7, after being frustrated with a number of Linux distributions (SuSE and later RedHat). The main issue I had with Linux was that it was difficult to download and compile external software. Pre-compiled software included in the CD distributions worked, but as soon as you try to use Linux software not specifically adapted to the version of the used distribution, compilation failed. I neither had the knowledge nor the ambition to solve every compile error for every program I needed. So I looked for something more standardized and robust. I don’t remember exactly why I quickly stuck with OpenBSD (I also tried Solaris), but now after using it (8+ years) I do …
Coherence. OpenBSD is built as a complete OS including the basic kernel, user land utilities and applications. This is originally why I switched from Linux because of the difficulty to use applications external to the chosen distribution. And Yes, Linux distributions are build the same way, but because of the vast number of distributions, I still believe compatibility issues are more prevalent.
It “Just Works”. The default configuration for the OS and tools normally works for most people with a basic setup. This makes it fast to get something working.
OpenBSD possibly have the best software packaging system on any OS (xBSD’s, Linux and MacOS included). OpenBSD pkg_x tools is the most robust and secure, simple to use (both as package user (1) and developer(1, 2)) and has the best upgrading capability. All relevant open source software is already available as pre-compiled packages, only a pkg_add away.
Evolution rather than revolution. OpenBSD is developed carefully in small steps keeping the stability and quality high at all times. This allows continuous improvement in quality and security which easily can be upgraded on existing systems. OpenBSD has been released timely twice a year in “a heck of a long time”.
OpenBSD security focus. The uncompromising security focus of the OS has many benefits apart from the obvious, to make the most secure OS around. Security focus drives quality, simplicity of code, simplicity of use and minimalism. Security is also getting more and more important on the increasingly hostile internet.
Secure by default. Default install of OpenBSD is already very secure with no additional configuration. Additional features and insecure configuration must be explicitly enabled by the user. In this way you don’t open up security holes by mistake.
Simplicity. Configuration errors is a common source for security problems. OpenBSD’s provides simple, clear and concise configuration files to avoid mistakes. This also makes configuration more effortless and pleasant.
OpenBSD culture and goals assemble some of the best developers around. You can always be confident that software released within the OpenBSD project has very high quality standards. Developers that work for a passion and take huge pride in their work will always be better than paid developers for commercial software.
Quality of documentation. In OpenBSD a false statement in a man page is taken equally serious as an implementation error. After all, what is the use of a perfect implementation, when you don’t understand how to use it.
Free. OpenBSD is the most free and open OS’s available. There are very few limitations on what you can do with OpenBSD commercially or non-commercially. OpenBSD is so free that even RMS envy it.
Versatility. OpenBSD could be argued to be mainly a server OS. OpenBSD will excel in almost any network related scenario. But lately OpenBSD has also become a very viable desktop OS. OpenBSD keeps its own distribution of x.org (Xenocara) and support all relevant window managers (KDE, Gnome, XFCE, etc.). Another sign of versatility is that the same base OS is used for all purposes. Compared with e.g. Linux, where there seem to be one distribution for each specific use case. OpenBSD also support a wide variety of hardware architectures and peripherals.
OpenBSD aims to be the best development environment available. This allow the developers to be more efficient and enable users to use or port the software they need on this platform.
OpenBSD is the underdog. Compared to other OS’s e.g. enormously popular Linux, OpenBSD is only used by a niche of users. At the same time OpenBSD has many merits compared to e.g. Linux. This makes you want to support the project in any way you can and enlighten people of its merits.
OpenBSD provides release song and artwork with every release. This is an obscure but adorable way to promote CD releases.