Allegiant is not there yet, but beginning to become a force for US to reckon with. They drive cheap fares to the markets that they serve and continue to grow their route map utilizing tier 2 departure cities. While they don't do connecting flights, they are building an impressive airline with hubs in Las Vegas, Phoenix-Mesa, and Orlando-Sanford. I have posted several times on the impressive growth of this airline. Their Q1 growth came from somewhere and I suspect US's decline is a primary driver of their growth.