Other top Spirit cities include Fort Lauderdale, Fla.; Detroit; Chicago; Atlantic City, N.J.; and Las Vegas, a compendium of leisure destinations with other carrier's hubs. In July alone, Spirit has announced new service from Minneapolis to three cities and from Dallas to eight cities.

A unique fare structure, which includes rock bottom fares and fees for nearly everything including carry-on bags, means that Spirit has not endeared itself to everybody. Often, bloggers and some travel writers are highly critical.

But Baldanza said: "Competitors don't like our prices and elitists in the media don't like that we unbundle."

Added Biffle: "If you talk to the vast majority of our upwards of 10 million passengers, they save money, stay in a nicer hotel, and eat a better dinner because of our low fares. We save millions of dollars a year. They like us."

On Tuesday, Spirit reported that in the second quarter it beat estimates and posted an operating profit margin of 16.3%.

"Some people have been concerned that Spirit is going into big city markets and competing against big carriers and could jeopardize their ability to keep margins up," said Imperial Capital market Bob McAdoo.

"Their expansion into DFW shows they can continue to do it," McAdoo said.

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