Flown to Trenton from Minneapolis a few times....only had one issue with the boarding process as one extra person somehow ended up on the flight. It was a ticketing issue and led to an hour+ delay and was fairly aggravating, but I'm pretty sure that could happen on any airline.
Personally, I like that they fly Airbus jets as opposed to regional jets, and considering every airline nickel and dimes you these days, paying for a checked bag doesn't make them any worse than Delta or others. I've heard horror stories about Spirit getting super-uptight with carry-on bags and things of that sort, but never had an issue with Frontier...I'll usually carry on a backpack and throw it under the seat for no charge.
They do charge for beverage service and things like that, but considering the cost and the Airbus planes, I'm very willing to fly with them.