Does anyone use espionage with great success?

[pchang]

buying the enemy

In Civ2, I used to buy entire empires (with the exception of the capital). The diplomat was the most feared unit in the game.

[daFrog]

Enemy Tactics

I actually had the opposite problem - every tech I researched the Japanese stole immediately (i.e. they had it the same turn when I tried to sell it to them immediately. Exception: When I built the TOE, which only gave me 2 advances for some reason, they only got 1 of them.)
Anyway - I knew they a spy because of this but I had absolutely no success in unmasking their spy ; in testing the limits of this, I ran through 6k in gold with no luck despite trying the various low-high chances.

I eventually had to take out the city where they had their Intelligence Agency. That stopped them (I think - or the war drained their coffers to the point where they couldn't do it any more.)