I've had Defcon for several years, and even tho I'm not particularly good at it I still really enjoy playing it.
For such a simple premise the game really captures your attention, and the visuals are good. If you've ever watched Wargames you'll feel right at home. The background soundtrack fits the theme perfectly - in that it's very dark and grim, sounds of people suffering from radiation sickness etc.
If you happen to own some Ambx lights then the game can make use of them (yellow warning flashes tied into enemy nuke launches , different colour flashes matched to the defcon status etc,...really adds to the atmosphere).
For the paltry few pounds it's for sale for now it's a no-brainer to grab a copy, even if it's just for the occasional game now and again.