Looking @ this article, it seems Spirit wins the award for best run airline in terms of a business in 2Q2011.

Lowest CASM in the industry (9.7 cents)
Lowest CASM ex fuel in the industry (5.41 cents)
Highest profit growth in the industry (69%, $10M -> $16.9M)

Spirit also has the 3rd highest load factor (85.9%) coupled with the 2nd highest growth (27.3% increase in ASM's), so clearly there are still plenty of folks who choose to forgo premium service for lower costs.