I can't speak for the earlier model water pumps, but in 1999, Ford changed the design of the pump, and there haven't been many problems. I have over 130,000 miles on the original pump, and have no plans to replace it until it fails.
-Mark-
'99 Mystique LS, totally original (including the original water pump and the premium stereo)! Bought new in December 1998 for $21,000.
208,000 miles