PROS:
Low fares
On-time most of the time
Many newer planes (though minimally outfitted)
Leather seats on many planes
Major hubs are in smaller, less-congested cities (vs. Chicago, Atlanta)
Excellent perks with their miles-earning credit card
CONS:
Competes heavily with Southwest; tends to mirror their budget brand of service/comfort (even FA uniforms are on the casual side)
No in-flight entertainment on domestic routes (except Hawaii)
Charges for snacks, luggage
Hideous change fee of $150 (don't book unless you're SURE you'll fly)
Overall, an acceptable airline, especially if you live near one of their hubs and can fly nonstop. I've never had a major issue with them, although it irked me that they charged for sodas. However, that must have bothered enough other people because sodas are now free.