We've used Frontier twice, due to the large price difference. Granted, it was a short (~1.5 hour) flight, so the comfort of the flight didn't really even cross our minds. We've used Allegiant in the past, too, but never Spirit. Haven't used Southwest (they don't service our area). Cheap tickets in exchange for charging for every bag, picking seats, etc. Hubs and I usually share the largest bag we can bring to cut down on the bag fee which hasn't been a problem, and we each bring a carry on (ie "personal item" which is free). But still cheaper than using the standard airlines in our experience. Fees were pretty clear when booking through the website (we usually "book" all the way to the point where you enter payment information, just to see everything added up).
Haven't had any problems with delayed flights or any other safety concerns. We would definitely use Frontier again, as they provide direct flights from "home" to a few of our choice destinations.