I regard the maintenance contract we have on our AC/heat units as a luxury I allow myself. We have two units. One is a conventional AC with elecric resistance heating and the other is a heat pump. We had both units installed in the year 2000. One year's maintenance was included with the purchase price. After that, I was spoiled.
The guys (our old friends) come out 4 times a year (we live in the country), change filters, check freon (add, if needed), and clean (wash down) the outside units. They charge about $475 total per year to do both units.
They say we get priority service and a discount if we need repair work.
I have had experience with gas, oil and radiator heating. I don't want them in my house. We do have two woodburning fireplaces.