Of the US "budget" airlines there are, I've only flown Frontier and JetBlue, and both seemed very nice. They have very new planes, so the likeliness of something going wrong is low. Something does seem dodgy about Southwest, though. Their planes are old and from what I've seen on their television show and heard elsewhere, they don't treat their passengers very well, either.
And the whole A,B,C seating thing is pure crap. I want to get to the airport at my leisure. I don't want to get their early just to get an A on my ticket so I can board first.