Everyone has their opinions, and each carrier has a dozen horror stories.
Generally, I've only had three bad experiences with Southwest (and I travel at least monthly w/ them for business). I just checked my account, and that's in 34 flights (68 one-ways) in 2 years!! I've been late for appts. only twice, and once I had a toddler who kicked my seat the whole way, and when I went to change seats, the flight att. wouldn't help me find a new seat, but that's it.
Also, I've flown USAir only three times in the last 18 mos. via LGA and each flight was late by, like, three hours. I seriously wanted to poke my eye out w/ a sharp stick!! Don't what was up with that.
My three Delta flights were on-time, if nothing special.
All in all, I prefer SWA. But it's "customer's choice!!"