I fly exclusively on another airline, which has a hub near where I live and on which I have high elite status. I may have occasion to book trips for a friend out of MCO. She is not necessarily as fussy as I am about upgrades, free bags, etc.

What do I need to know before booking on Frontier? I think they charge for a carry-on, right? Is it significantly cheaper if you pay for it at the time you book the ticket?

What I'm used to on American is:
Basic Economy - no changes or refunds, board last, extra charge for checked but not for one carry-on and one personal item, you get whatever seat they give you but you may be able to pick a seat after check-in opens 24 hours before the flight.
"Main Cabin" - same for bags, seat selection allowed but many seats are blocked off for an extra charge, board not-quite-last, no change fees.

Frontier seems to have a discount plan, $60 for a year, and you get lower fares - is that correct - any other hook to that discount plan?

What's the onboard experience like? Is it just flight-ordinary-bad, or do they find ways to make it worse?