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?

A checked bag is cheaper than a carry-on. Yes much cheaper if you pay for the bag in advance. Stretch seating (Economy plus type) is nicer if it is a long flight.

I've flown on numerous Frontier flights and don't find the experience much-different than any other carrier, except for Delta where I have status. Yes, a checked bag is cheaper than a carry-on bag. The discount plan typically only saves about $10 per ticket yet it can be more. But I think the member has to be flying (not positive about that) and then the member plus companions on the res get the lower price. Some people complain that the seating is "harder" but it's not a deal breaker considering the price compared to the competition. And while you can't recline the seat (because they're "pre-reclined" lol), the reality is that the person in front of you can't recline their seat either. Discount or no discount, they can have some amazing prices. I've flown from Orlando to Cleveland for $15ow and to San Diego for $17ow. They have constant sales.

It's better than Spirit.

That could be the new slogan.

They've cut out all drink service except $2.99 water bottles. You had to pay for it before (e.g. even a diet Coke with ice was $2.99 for the whole can). But it'll be nice when they can bring that back, and on LAS flights they'd get a few people spending $30+ on beer/booze multi-quantity deals, which I'm sure made them some money.