I use them a lot, to weaken garrisoned units before attack, with both cavs ands rifles.
They don't cause so much damage as siege units (damage is in fact capped), but their range is very wide, so you can often attack several cities from same base. Up to 4 airships can be based in a city (of yours or of an AI you have open borders with).
Usually, when going to total conquest of an AI, I have 2-4 groups of 4 airships in border cities. After AI city is taken, in the turn cavs move to next city, rebase airships to newly conquered city, in order to be near to next targets.... and so on.
On each attack, airship reveals a great deal of fog, so it's very useful with cavs to attack units and steal workers two tiles away.