Their CLE ticket sales have supposedly been stellar, at least at their introductory fares.

tk said
    In the long run we need better airlines than Frontier for the airport to prosper ...  Frontier is fine but we need more and other also or we will be in long run decline.

Back when Continental had just started its buildup at CLE, the Plain Dealer (in one of its 'poor pitiful us' funks) carped that when CLE finally got a hub it was by the worst airline in the country, which may have been true at the time, but times changed. Maybe they will with Frontier also, but - by design - Frontier will remain a low-fare, low-service operation.  I think that's good for CLE in (re)building its passenger count as much by pulling people out of their cars as by stealing traffic from the other carriers.  I fully expect Spirit will arrive in CLE within a year, too.

Both Spirit and Frontier have a BUNCH of aircraft on order. They will be on every major route within a couple of years.  The big guys will have to provide a markedly superior service and a level of reliability that warrant the higher fares they charge (something they don't do now) or else they'll have to match the Spirit/Frontier model.