The original plastic fin water pumps tended to break, but once you replace it with the metal fin water pump they last just as long as any other car. The Boxster has its problem areas but a $400 water pump is hardly a reason not to buy such a fun car.
If you want to worry, go ahead and worry. I'll be out enjoying my Boxster while you do.
I think this sums it up nicely:
http://sites.google.com/site/mikefocke2/problemareas