As SLF- I fly the low-cost carriers 60-80% of the time. Why? I typically get better service for my money. They are friendlier and more flexible. Just this week I flew AirTran (nee Valujet). The A/C are new, the cabin crew courteous, and I'm not disappointed in not getting a meal- I wouldn't get one on a major anyway. As a matter of fact, I changed my itinerary and they didn't charge the change fee and they bumped me up to Business "First" Class (although the cast on my leg and the light 9/10 travel load may have helped. I later noticed the counter agent working the pushback. This is compared to Delta where the get you for every change and standby fee they can. So why would I fly Delta if I don't need too? Requisite Caveat- I know a lot of good Delta people- I know it's the management.
Second issue-safety. The Valuject crash was a chain of events- possibly due to low paid poorly trained people- but it certainly doesn't happen at the lower-fare airlines alone. There are plenty of incident reports tagging everybody-if you know where to look.