When I first played uplink I was amazed, but after a while the tension is gone, you know how all the software works, you realise the game is quite small in scope, and it becomes obvious that it's only a game and doesn't come close to realism. Brilliant first effort from a small developer, still.
Defcon is just great, it doesn't do for me what I'd usually expect from a game, like provide constant variety and keep me glued to the screen for hours, but it's almost in a different box from "games". I don't play Defcon when I want to play a game, I play Defcon when I want to play Defcon. It's a tactical outlet. When you're over the atmospherics and cool stylings, it becomes a very bare and potentially deep set of rules for you to compete with people. Kinda like chess, but less geeky and boring. You don't play chess to look at the pieces, or to come up with interesting and new things to do with them.
Darwinia is a great, quirky game. For me though, it's where IV went from indie, niche, "difficult to approach" games, to putting one foot in the mainstream. That's not a bad thing, but if I want mainstream games, there are thousands of well produced mainstream games out there, Darwinia luckily has more to offer than that but IV's limitations at the time, and their smaller scope compared to developers with masses of money shows itself a lot more with this game. When you're done grinning like a kid at the awesomeness of the concept and style, there are pangs of realising it would have been bigger, better and more solidly built. The engine is very flaky. With games like Defcon and Uplink you don't need to worry about that, you know what you're dealing with, they're small, beautifully realised, focused pieces of pure gaming bliss, without too many unnecessary bells and whistles. They do what they do, that's what I love about Introversion games.