JASON There is an installed base and a rather impressive one. Apple uses FrontBase in many of its user-driven forums as well as other internal functions. Many other corporations use it, too.
FrontBase is very, very cool and offers some security options not found in other sql-based database engines. It has a tool to help convert from other db structures inclusing FileMakerPro and a pretty good support system if you need it. We have been looking at replacing MySQL with a few FB servers, but have not had the budget yet to do so, but it is the plan.
(Version 4.1.15)