Defcon is EXTREMELY hard. You get owned the first few times you go online.
Uplink is an incredible game. It's soooo adictive and tense. To keep longevity, though, you're going tp have to download mods for it, like FBI mod or a LAN expansion mod. It gets a bit samey if you play it too much, it's a shame Introversion didn't expand on the LANs.
However, still an awesome game.