I first used Subversion about five years ago while writing my second book. It was an early milestone, but it was easier to compile, install, and configure than CVS was, and it was much more powerful than RCS. Since then, I’ve used Subversion to host every book I’ve written or edited. (See The Making of BSD Hacks.)
Though distributed version control systems are gaining in popularity (and though I use SVK atop Subversion), Subversion is still tremendously useful. Try being a F/OSS developer without knowing how to work with a project in Subversion, for example.
The preponderance of support with free hosting providers for F/OSS projects, as well as the quality of documentation and implementation, mean that I spend a lot of time working with Subversion. I’ve rarely had any trouble (even as far back as milestones 19 and 20). My work would have been much more difficult without this project, so thanks to everyone who’s contributed in any way.