Just to state the obvious....a ground to pipe is only useful if that pipe is metalic all the way to the outside of your house.
Connecting to a plastic/non metalic pipe is useless as a ground. Just because the pipe you see is metal does not mean it's a good ground. Some houses run all their water thru a plastic filter (softener, iron, whole house,....) and if there is no all metalic path to the outside of your house....it's not a ground.
If your water comes from a well and you have plastic pipes outside (or down the well), it's not a ground either.
Most people forget all of the above and just figure...ah a pipe, I'll connect and I now have a good ground. What you have is a connection that can potentially kill you, not a ground.