Frontier/Spirit/Allegiant are all great for what you pay.
They fall greatly below Delta/American/United when there are irregular operations, bad weather, mechanical, etc.
They fly a much leaner fleet so if there are issues they likely do not have a plane or flight crew to substitute when there are issues.
They also suffer from a reduced schedule. Often times there are only 1 or 2 flights a day from some cities so if there are issues it might be the next day or later before you can be accomidated.
Having said all that, my wife and I fly Frontier and Spirit often. It is hard to pass up some of the amazing fares. We have two wheeled carry ons that meet the sizing criteria for a free personal item on Frontier and Spirit and often plan 2 or 3 day trips around $17 dollar fares.
You will have a great flight.
