Ah yeah, the "D" series of games is very unique, but not for everyone. Very strange and somewhat hard to appreciate games, like Illbleed (which is not an RPG).
D2 and Maken X (which is also not an RPG) are the only Dreamcast games that I know of that cannot be properly burned and played freely on the console, so in a few years these may become some of the rarest games on the platform. They are worth checking into for collectors, at least.