I pretty much agree with mr pogo, but I feel the need to expand on some things.pros-
slight hp gain
easy to change out (after initial mods)
supposedly flows more than stock, at least at lower rpms
less leakage prone
spare can be kept in trunk
wp motor can burn out if adequate wiring and relay not used
wp motor konks out, engine heats up very quickly
...mine has been on the car for 11 years which has to be more than the 3k hr the HD spec is. Have had a spare in the trunk for 10 years...have not needed it., I've had two Meziere HD water pumps fail on me, at the most inopportune moments (though they both exceeded their hour duty ratings), but no engine head damage.
I'm on my third one, and very happy with it.