Double Up When Buying Delta Miles

US Airways’ recurring 100 percent bonus for purchased miles has been among the year’s most lucrative frequent flyer promotions.

With its new buy-miles offer, Delta has taken a page from US Airways’ playbook, with similar but not identical results.

Offer Details

Through December 31, SkyMiles members who use a Delta SkyMiles credit card issued by American Express to buy miles will earn a 100 percent bonus on the purchased miles.

SkyMiles members may purchase a maximum of 60,000 miles per year, which translates into 120,000 miles with the bonus.

Delta miles are normally priced at 2.8 cents each, plus a 7.5 percent federal excise tax, so 60,000 miles will cost $1,806, including tax. With the bonus, you can buy 120,000 miles for that price.

Deal or No Deal

There are two key differences between this Delta promotion and the similar offers from US Airways.

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The first is the requirement that the Delta miles be charged to a specific credit card, whereas the US Airways offer made no such demand. That’s not an issue for anyone who already holds a SkyMiles credit card. But for those who don’t, this offer is effectively a sign-up promotion for American Express. Do you want another credit card in your wallet?

The second difference is in the rewards currency—all frequent flyer miles are not created equal. In particular, the value of any airline’s miles depends in large part on that airline’s policy on award availability. In other words, when you go to redeem the miles, how hard is it to find award seats to desirable locations?

There’s no final word on which airlines deliver the best value, awards-wise, but Delta has come in for considerable criticism this year, based in part on a study that found SkyMiles award seats available just 12.9 percent of the time. The same study ranked US Airways even lower, with a 10.7 percent success rate in its test bookings of award flights.

Nevertheless, spending less than $2,000 for enough miles to fly to Asia or Europe in first or business class is definitely a noteworthy deal—if you can book the award flights at Delta’s so-called “Low” availability prices. If award seats are only available at the “High” prices—185,000 miles each way for a premium seat to Asia, for example—the value of the offer quickly deteriorates.

This is, in short, a potential deal.

Reader Reality Check

The value of this offer ultimately depends on the availability of SkyMiles award flights. How successful have you been in redeeming your Delta miles for free flights?

This article originally appeared on FrequentFlier.com.