I love Espionage in both CiV and BTS, though I will admit that the BTS spies were a lot more entertaining - I enjoyed sending them across the map - seeing them sneak around and blowing up my enemies' oil resources

;.
I also enjoyed using them for exploration or for converting culture and fomenting unhappiness (especially in the Civ 1000 AD mod with Europe so tightly packed; I loved messing with Barcelona, Cordoba, Bremen, Ribe, Salzburg, and London as France and eventually getting them to flip!
I do like the "Steal Technology" and Intel actions in CiV, but I detest the fact that I can't move my spies about the map and wreak havoc! I have to gather intel, rig elections, and steal tech - kinda boring after awhile, especially when it takes 42 turns to steal a technology most of the time. It puzzled me as to why, in G&K, they removed the spy movements - how is he supposed to travel from one end of the globe to another with no line of sight? Despite the fact that reality is obviously not going to come into play in a game, I do wish some of the extra spy actions had not been shorthanded and taken from the game.