I bought a Frontier ticket for my GF due to a last minute trip change once last year and it was "fine." They charge extra for bags, but it's cheaper to buy "the perks" package which includes priority seating/boarding in addition to a carry on and checked bag (if you have both a carry on and checked bag, which she did). She sat right at the front of the plane and was "priority bitch" and treated like royalty by the staff, despite everything about it still being a budget airline. She said it was extremely strange.
I looked into the membership and it didn't pencil out at the time, but I could see it making sense in different circumstances.