What they get in return is unsurpassed access to priority check-in lanes, first-class lounges and free upgrades.

CNN: The fabulous lives of air miles millionaires

So far, airlines say travelers end up spending more for upgrades in online auctions than they would spend at check-in.

WSJ: El Al, Virgin Atlantic Are Airlines That Auction Upgrades to Business Class

However, the websites of airlines or hotels are likely to prove more popular with frequent travellers than those of online agents because the former can also be used to check up on rewards, cash them in and to find special offers, such as upgrades.

ECONOMIST: The travel industry