I've used Interbase. Don't install it on Windows because there is no way to uninstall it (or, at least there wasn't with the version that I tried). Other then that, and the fact that it stopped working for no apparent reason, it performed decently. The JDBC connectivity, however, wasn't wonderful. It required a separate connecter application that tended to use up the majority of Windows 98's TCP stack, making it difficult for other applications to establish TCP connections. I wouldn't recommend it over MySQL.