I voted unrealistic but then on reading Exile Ian's post I have to qualify:
As a game that attempts to model history, it contains a lot of inherent realism in the tech tree, military units (okay it's not perfect but IMO the civ games have slowly been improving on this.)
The unrealistic part is the basic premise that terrain resources can be used endlessly without ever degrading or running out - except for the strategic ones, which in reality tend to be very abundant! (Coal, iron, oil, etc).
Speaking of RISK, chess, and other board games: anyone out there familiar with the CIVILIZATION board game? Predates Sid's CG's, and is much, much simpler, but very fun with the right crowd.