From September 1 through October 31, folks like me who still have a Diners Club card earn a 25% bonus on transfers of Club Rewards points to AAdvantage.
Meanwhile through end of September transfers of 40,000 – 59,000 Diners Club points into Delta Skymiles earns a 20% bonus and 60,000 or more points earns a 50% bonus but that offer holds little interest to me.
In both cases my inclination is to save points in Club Rewards for their flexibility rather than being tempted into a transfer just for the bonus. Though Diners Club has lost several partners in recent years, predominantly because they’ve become a Mastercard and compete in the US against airlines’ own co-branded card offerings now (not to mention the card issuer earning a lower interchange fee as a result of the switch, which also obviously gets the card much broader accepance), partners still include Aeroplan, Alaska, South African, and others. It’s long been the only card positioned to earn Hyatt Gold Passport points, though Chase will be coming out with a Hyatt Visa shortly.
That said, since it’s now widely expected that American Airlines will be discontinuing their practice of counting miles earned from any source towards lifetime elite status qualification at some unknown point in the near future, folks with lots of Diners Club points and an eye of lifetime Gold (1 million miles earned from any source) or lifetime Platinum (2 million miles earned from any source) in the AAdvantage program might consider taking advantage of the bonus now.