An intelligent investor wouldn't hold airline stocks for the long term. That said, they can be excellent trading vehicles as they tend to be relatively inversely correlated with the price of oil. When the overall market - and oil in particular - was getting pummeled a few months ago, airline stocks weren't just holding steady - they were moving up appreciably.
I like that there are, effectively, a wide variety of macro bets being made within the sector. Do you think that oil is moving significantly lower? LCC - the only major airline that doesn't hedge its fuel costs - might make a lot of sense. Do you think that merger integration, cost cutting, and fuel hedging is the key to short-term success? You should look at UAL. Do you think that owning your own refinery - another way of trying to limit your fuel costs - is the way to go? DAL might be the one for you.
At the moment, given their prices, the economic outlook, and the price of oil, I'm somewhat neutral on the airlines. That said...if they dropped 20-30%, I'd start looking at them seriously once again. You won't make much - if any - money holding them for a decade, but you might make quite a bit of money if you're smart enough to buy them when they're low and sell them when they're high.