Clearly you're not alone as evidenced by their financials. I have flown WN and like the flexibility of changing tickets without any fee and the 1 way awards. The flight experience is fine, but I've only taken short flights. Not sure how I would fare on a transcon.

Upgrades, international flights, priority security lines, priority boarding, better flight schedule from LAX, priority seating, priority phone service, and just plain knowing the system are some of the reasons I am loyal to a full service or non-profit airline (in my case UA).