Best and worst airlines for frequent-flier programs

Among U.S. carriers, Southwest has the most seats available, at 99.3 percent. JetBlue was next, being able to accommodate requests to redeem miles nearly 80 percent of the time. The worst U.S. carriers: US Air and Delta, which didn't have requested seats three out of four times, according to a study by IdeaWorks. This summer will be particularly tough because the airlines are heavily booked (SF is the hardest city to get into and out of using frequent-flier miles). From the WSJ:

The research unearths a tip for travelers having trouble using their frequent-flier miles: It might be easier to get "free" tickets using miles on low-fare carriers than it is on big carriers with global networks. The survey showed that six low-fare carriers had seats available for 85.6% of the requests made. The 18 other carriers in the survey offered seats for only 62.9% of requests made. One reason for this is that low-fare airlines tend to have smaller credit-card tie-ins. American, by contrast, said that 62% of the 185 billion frequent-flier miles it issued last year went to customers as credit-card rewards, not flying in airplanes.