The harmonic balancer is a part that should be inspected everytime you take off the engine cover. The part is made up of two metal pieces with a rubber middle. The rubber absorbs vibrations which is why it is called a harmonic balancer. After years of service, the rubber will break down and fail, which causes the two metal pieces to rotate separately. Eventually the pulley will fully separate and strand you. The easiest way to inspect the pulley is to scratch some lines on the pulley that goes over the rubber. If the lines do not line up after the next inspection, the rubber is failing and the two halves are starting to spin separately. You should also closely inspect the rubber for any damage.
With my car, the pulley had not started rotating separately, but the rubber was looking pretty bad. I would rather spend money now than spend more money after it fails.
In addition to replacing the pulley, there are some oil seals that should be replaced at the same time. The oil pump o-ring and the crankshaft seal can be easily replaced once the pulley is removed.
Overall this project is not very difficult. The biggest challenge is acquiring a long enough pry bar to break loose and tighten the pulley bolt. It is around 250 foot pounds. You can get a various lengths of pipe from a hardware store, just make sure the hole is big enough for the ratchet to fit in. You will also need a long 1/2" extention to bring the ratchet out of the wheel well.