A lot of this comes down to "What do you want from the program/airline?" For me, VX wins my business because they offer a consistent, reasonably high-quality F hard product and a fine soft product on all routes, rather than on just a few routes (e.g. NYC/WAS-LAX/SFO), meaning I can often derive more benefit from heading to Florida and catching a flight from there (or from diverting through DAL)...and in almost all cases I still get a respectable meal on each flight in the process (something that I'd note has gotten dropped on at least one carrier...I want to say Untied or Deltoid...on everything but their super long-haul domestic flights). The exception is the occasional super-short-haul flight (LAX-SFO, I'm looking at you...).

With the other airlines out there, I've seen far too many cases where you end up on an off-brand flight with no improved class of service available (and/or have to pay an annoying amount of attention to what plane you get). For me, Virgin gives a stunningly straightforward product, doesn't play round-trip fare games for the most part...really, it's the easiest airline to work with in many respects IMHO.