Consumers with busy fall travel plans have a shot at unlimited travel at bargain prices this fall, but they ll need to act quickly.

JetBlue Airways (JBLU) has brought back its All You Can Jet pass, which allows customers to fly coach on any of the airline s routes as often as they like for a month. Travelers can choose between a $499 pass that excludes Friday and Sunday travel, or a $699 pass for travel on any day. Both passes are good for travel between Sept. 7 and Oct. 6. They include taxes for domestic flights. Passes are available until Aug. 20, or until the airline sells out, whichever happens first.

The pass was first offered in August 2009 for $599.

Last year it was pretty well received, says Rick Seaney, the chief executive of pricing site FareCompare.com. (Passes sold out in two days, and reports from travelers were positive.) With fares up 20% compared with last year, it s still a very good deal, he says.

Manipulating capacity by filling up cheap seats allows airlines to increase fares even in times of slow demand, says Basili Alukos, an analyst who covers the industry for Morningstar. He adds that by selling off more coach seats, more travelers could be driven to spend on business class.

Of course, the JetBlue pass has competition. With plenty of fall fare sales out there, travelers will need to plan out their itineraries carefully to assess whether the pass is right for them. Here s how:

Compare prices

The simplest way to check if either of the two passes is a good fit is to compare the pass cost against individually purchased ticket prices for planned trips, Seaney says. Assess fares on competing airlines as well as JetBlue to account for sales. Generally, flying coast-to-coast twice is your break-even point, he says.

Friday and Sunday exclusions are typical of fare sales, so fliers considering the $499 pass should pay particular attention to ongoing deals, he says. Southwest (LUV), for example, has a sale through Aug. 19 for travel between Sept. 7 and Dec. 14. Its Boston/Chicago fares start at $114 each way, while JetBlue s range from $94 to $160.

Join the club

You must be a member of the free TrueBlue program to buy a pass. Pass-holders earn 3,000 to 4,200 miles for buying the pass, but they do not earn miles for flights booked with it.

Calculate total cost

If you re going to fly out of the country, the out-of-pocket cost would be a little more than the pass prices indicates, says Ed Perkins, a contributing editor at advice site SmarterTravel.com. You have to factor that in. A passenger flying on a pass from Los Angeles to Aruba, for example, would still pay $103 in taxes and fees, bringing the total cost of their pass to $602 or $802.

Even for domestic travel, consumers should factor in the prices of hotel, car rental and food and activities during your stay. The destination costs are going to be considerably more than airfare in any case, Perkins says.

Pick the right pass

Business travelers and leisure travelers with flexibility are likely to get the most value from the cheaper $499 pass, Seaney says.

Be realistic

Once you know how many trips it will take you to break even, assess the likelihood that you can follow through. It sounds like a really good idea, but the problem is, I don t know that many people who could or would even want to take that much time off to do nothing but fly, Perkins says.

Limited JetBlue hubs may thwart even determined fliers. San Diego residents, for example, would have to fly through New York to reach any other destination the airline serves.

Check seat availability

Booking for the All You Can Jet promotion starts Aug. 23. Travelers must book at least three days out and make arrangements through http://www.jetblue.com/aycj. Travelers who don t have much flexibility in their travel dates may have a tougher time getting seats this year because most flights are already operating at close to full capacity, Seaney says.

Watch for penalties and fees. Customers who cancel an All You Can Jet flight within three days of departure must pay a $50 fee. Failing to show will cost you $100 and the use of your pass until you pay the fine.