I'm a Southwest guy, mainly because they offer a ton of flights out of Nashville, and their freebie policy was sooooo good in the early days when you bought on the net. (Buy four flights, get one free. Sadly, this is no longer the case)
I liked them in the early days because they were innovative and fun, and didn't take the fact that they were an "airline" too seriously.
Even though I'm still a big fan, they've changed some over the years, due to their success. The flight attendants aren't nearly as entertaining as they once were, but for the cheap fares I can live with that.
They also appear to be an excellent company to work for. A friend of my wife's has been a flight attendant with them for a half dozen years or so, and he LOVES it and makes pretty good money too!
I was a NorthWest fan for a long time, until on successive weekends I ended up stranded in Minneapolis one night because I couldn't get to Missoula, Montana and the following weekend I spent most of a Sunday trying to get out of Philadelphia back to Nashville. My big problem was that they wouldn't TELL us anything. In fact, after the two weekend problems, I wrote the President of Northwest (at that time) a long letter, stating just that. I was courteous, and said no one would object to problems if they would just trust us to understand WHY there were problems. I got a nice letter back with some $5 airport food vouchers and 2500 frequent flyer miles!!!! (And I was thinking I might get a free ticket! )
Haven't flown JetBlue, but I hear good things about them too.