Mercurial fills a gap caused by the learning curve that has been left aside by most version control systems. I started to use it just about 3 months reluctant cause some non positive experience with other VCS, but it exceeded my expectations. Easy to use, intuitive, and lightweight, in part because it does not necessarily run as a service on the machine.
Now I'm working in medium projects (web applications/CMS/ERP, etc), and in all of them I've choosen to use Mercurial as our primary VCS.