I rarely use spies, because spending the gold on research is a much saner approach, usually.
Anyway, some spies actions are quite powerful (poison water supply for example can half the population of a size 8 - 12 city. AI used this against me once, not very enjoyable). Also, destroying a tile improvement is another good use (eg in wartime, depriving the other side of their strategic resources). There's a nice thread on this at
CivFanatics. (Here's
another one.)
Edited by Voskhod - 22-Oct-2008 at 11:29