Overall, I would give Frontier/Early Return an A-. However, it would be tough for me to consider using Frontier if I lived in a different city with a major airline hub. To avoid repeating too many of the great points above, I will just submit a list of pros and cons:
Pros:
50% mileage accrual bonus, free cocktails for Summit members
Easy elite qualification. At around 40k/year I would not qualify for much elsewhere
Great service at DEN
Nice planes, IFE. A better flying experience than UA E-, which is all I'd get for the first year, so I can't justify switching.
Plenty of non-stops from DEN
Friendly, prompt, US-based customer service
Cons:
No Europe or Hawaii rewards
Fewer partners for accumulating miles
No first class seats
Airport service away from DEN spotty