B-in-law used them last month- only catch was St Pete airport and we live in Tampa- extra time to/from airport. Plus he had no checked luggage. Last year sister used really cheap Spirit- I think they paid for carry on luggage. In the past Allegiant has had a poor maintenance reputation (to St Pete) but buying new planes so maybe their reliability will improve.
I agree about winter direct flights- son once had a late fall (Nov or Dec) Seattle to Tampa via Phoenix red eye that was delayed because the airplane was caught in New York snow before heading to Seattle (who knew that would be a problem???) , missed connection by a couple of hours, finally got here hours late. Going home via Minneapolis was no problem. Thank goodness he’s taking Alaska Airlines this trip- there’s been snow up north already.
That’s a yes to Frontier currently.