B

Pros
Redemption options start at 12.5k miles for a OW flight
Miles bonuses with only slightly more expensive tickets
No miles penalty for booking a one-way flight
FF status is relatively easy to obtain (20 one way flights for Ascent) and has decent benefits
250 mile minimum credit on all flights

Cons
No FF alliances mean that miles can only be redeemed for Frontier, which has limited international options
Program used to be MUCH more generous (10k miles for a OW ticket, 15k for a R/T)
Credit card program requires an annual fee to get any sort of reasonable benefits, and even then miles don't accumulate very quickly
Higher-demand flights cost 22.5k miles for a OW, even if pricing on the flight is less than a longer, non-peak flight

I've gotten award flights from Frontier before, but they were all before the airline adopted its current award tiers (12.5k OW, twice that for round trip). If I flew them exclusively I could probably hit Ascent status by the end of the year but shortly after that point I'll probably go back to TX, and DEN and MKE aren't on the way to most places I want to go.

As an airline, I prefer Frontier to American and Southwest (the only airlines that I've used for more than four segments). However for me their rewards program just isn't that amazing.