If you own a Delta SkyMiles Credit Card from American Express, the odds are good that you book frequent flights with Delta. After all, you earn free miles with every purchase that you make with these cards. The SkyMiles cards are typical American Express products: They offer generous rewards programs and several perks.

When it comes to redeeming the points you earn with these cards, though, there’s a problem. And it’s not the fault of American Express.

You can blame this one on Delta. Search frequent flier forums across the Internet and you’ll read the same verdict: Delta’s Award Redemption Calendar is nearly impossible to use.

If you want to see just how frustrating the center is for users, just visit this thread at the flyertalk forum. In it, a user discusses in detail how consumers can find low-tier rewards when they’re redeeming their points. The problem is, users shouldn’t need a detailed guide full of tricks. Finding low level rewards should be as simple as selecting the “Low” level when you’re searching on Delta’s site.

Unfortunately, what usually happens is that Delta’s calendar instead switches to the medium or high rewards levels. This can be incredibly frustrating for users.

You can also read about the problems that users at having with Delta’s Award Redemption Center at this blog post at Ask Mr. Credit Card.

Sign of the Times?: Of course, this might merely be a sign of the times. Cardholders have voiced complaints about nearly every airline’s frequent-flier program.

The Wall Street Journal last summer wrote an interesting story about this. According to the story, airlines are making it more difficult for consumers to convert their miles into free seats. The paper cited a study showing that two of the country’s largest airlines — Delta Air Lines and US Airways Group — did a terrible job of awarding seats to consumers who had earned free flights.

Delta, for instance, had awards that required the lowest mileage available for only 12.9 percent of requests, while US Airways brought up the rear with such awards available for just 10.7 percent of requests.

On the opposite side of the spectrum were Alaska Airlines and Southwest Airlines. The study found that Southwest was able to fulfill 99.3 percent of consumer requests for awards seats that required standard mileage levels. That led all carriers. Alaska Airlines was able to provide seats on 75 percent such requests.

Is It Time To Avoid Miles Cards?: Many critics have advised consumers to avoid mileage credit cards altogether. It’s simply too difficult for consumers to redeem their points, these critics say.

And it’s hard to argue.

Just looking at the Delta Award Redemption Calendar should be enough to swear anyone off of miles cards for good.

There’s also the fact that credit card issuers offer a host of rewards cards that let you cash in your points for a wide variety of items, including airline miles. With these cards, you’ll earn points, usually with every purchase, that you can eventually turn into gift certificates, cash rebates on your credit-card statement, airline miles, hotel stays and car rentals.

Why not explore one of these more flexible rewards programs? Unless you are the most frequent of fliers, your favorite airline’s credit card might not be the best choice for you.