I've flown enough domestic flights to rack up elite status on Delta, Continental, American and USAirways. My opinion is that a flyer can get good and bad experiences on every airline. I used to think that Northwest and USAirways were terrible, but in retrospect, many "bad" experiences were due to reasons beyond any airline's control, or due to the airline industry in general. Each experience is tempered by the individual airline employees and you get all kinds in every airline 🙂
Certainly, one can make generalizations for certain airlines. For example, I've found that USAirways have older dirtier planes, which is consistent with the airline's financial problems. Flying Delta through Atlanta is always frustrating due to the congestion at that airport. I've run into both good and bad employees everywhere, as well as assho passengers who don't understand rules. I won't fly Southwest because I don't have the time or inclination to be present at the airport early enough to snag a good seat. (Reserved seating is a must for me.) I like Continental's elite flyer program. Low cost airlines have torture seating and often have limited schedules so if you miss a flight, or if one is cancelled, there are few alternatives. And then there is the matter of hub airport quality; e.g. Detroit is awful; Atlanta is OK, Charlotte is pretty nice, Miami--Ugh, Philly and Pittsburg are cool, Newark is improving . . .
I've flown JetBlue only once and it was great. Of course, the plane was brand new, had a TV for every seat, and was only 1/2 full.