We've seen LCCs like Air Tran, JetBlue, Spirit trying on new international routes to the caribbean and latin america.

International routes could bring many challenges, not for the flying itself, but for the simple cost model of these airlines. So, how are they performing on these routes. Are they getting good yields?

I wouldn't say Spirit is 'trying on new international routes' - flying to the south out of FLL is Spirit's bread and butter business (around the time US Airways abandoned their FLL 'Central/South America hub' plans, Spirit started theirs).

As far as financial performance on the routes, don't know for sure Spirit's privately owned, and Air Tran and jetBlue don't break out their data on their international routes, but competition is lower so margins can be higher, especially for a carrier with lower costs such as Spirit, Air Tran, or jetBlue compared to their competition, primarily the US legacy carriers and the national airlines of various countries, who are new to the game of competing without heavy government assistance. Air Jamaica, the largest Caribbean carrier, is a prime example. That being said, the Caribbean is a leisure market, so demand is dependent on the economy. Money's tight right now for everyone, so I'd imagine the yields are down too, though they're still likely higher than overall US domestic yields are.

Southwest has a FLL 'focus city' as well as a large presence in the state of Texas, so, it's likely only a matter of time before the competition heats up in a market that is as old as international commercial aviation itself (Miami City Hall is Pan Am's old flying boat terminal).