We have had at least 3 bad customer service events with Frontier where they didn't care about doing what was ethical for the customer, and seemed to only cared about the bottom dollar and their own best interests. I will offer the two of those that I remember most clearly.
First intro to Frontier a few years ago--we were stopped in DIA by a Frontier credit card salesperson who offered us a "free ticket" for signing up for their credit card. Well, they didn't actually give us a "free ticket," as advertised. They gave us 20,000 miles, which at that time was enough for a free ticket. That very month, they changed their FF mile requirements to 25,000 miles, which meant that we now no longer qualified for a free ticket (and were not "grandfathered" under the old rule even though we got the miles under the old rule), despite being promised one. We contacted Frontier Airlines customer service (if you can call it that) and they told us "too bad...that's now the requirement and there's nothing that can be done." We should have run then, but we stuck it out and are sorry we did for other bad customer service issues throughout the last couple years. THEN...
Latest problem...they sent us a notice in October, 2012 (that we still have in our inbox), that our FF miles would expire in March 2013. We went to use them this month and they told us our miles expired in December and we should have received some kind of notification with the change (we didn't) and there's nothing they can do about it. My husband spent an hour on the phone with them explaining the unfairness of the situation and they basically gave us the finger and said, "Too bad suckers!" I sent an email to their customer complaint link last week and got some lame reply that they are backed up on complaints and don't know when they will respond and that we are free to call their helpline for a quicker response (the same help line that gave my husband a runaround for an hour and said we are out of luck).
Today (2/4/13) I posted a complaint on Frontier's Facebook wall today but they removed and blocked my post within 5 minutes because they don't want anyone to see it.