Evil Genius

[Boohugh]

This interested me when it first came out (I remember playing the demo) but I heard it didn't have that much replayability and got boring quite fast so never got it. I guess as you've only just started you won't have met these issues yet but has anyone else played it and got bored or did/do you come back to it? Additionally, is there a sandbox mode or similar, or are you forced to play through the missions only?

Edit: Oh, and does it work with Vista as it's fairly old now?