Well, for starters I was lucky, because Northwest, the airlines I usually travel, was cancelling flights right and left. No such problem with Sun Country. In addition, we flew from the Humphrey Terminal at MSP, which is considerably less busy than the main Lindbergh Terminal. Overall, Sun Country was not quite as slick and cheery as NWA, but their planes were clean, employees polite if informal, and they were generally reliable and on time. We did have one delay waiting for a plane to arrive, but that's happened with the majors, too.
In a time where airlines are scratching for every buck, Sun Country was marching to a different tune. They charged a fee for pillows and blankets, which is a little odd. But they also gave you a free light meal on a 3.5 hour evening flight, which no one else seems to do anymore. And had leather seats too!
They now have their own frequent flier program, too. So that may have removed the main reason I've stuck with NWA- the opportunity to earn free flights. If I can keep using quiet little charter terminals, and avoid the big check in and security lines, I'd fly Sun Country and other non-major airlines regularly. The time and hassle of security and other lines alone that we avoided was worth it. I was pleased with the experience and would highly recommend it.