My wife and I flew DFW-DEN-DFW 11/13 - 11/17. Was my first Frontier experience on one of their A319's with the new seats. Some thoughts:
1. No TSA pre-check. Not that big a deal to me, though.
2. Seats did not recline. We had 17E and 17F.
3. I am 6'2", 215 lbs. and I had about 1 inch to spare of knee room. For the about 2 hours total on the plane I was not uncomfortable. With over 50 years of chronic lower back problems, I was concerned, but the flight turned out ok for me.
4. I used an LL Bean messenger bag with 5 days of clothes for me and it fit under the window seat in front of me. Gate agents and FAs did not even look at it. My wife carried a Red Oxx Extra Small Aviator bag. Her bag fit under the middle seat in front. No worries there.
5. In our case both the outbound and return flights landed 5 minutes ahead of schedule.
6. Little tray is just about useless. No way it holds a reasonable size laptop. Built for small tablet. But, that is what you get for such a low fare.

I would fly them again at their low fares. Coming back from Denver, we had the last Frontier flight back to DFW. Had that flight been cancelled, we would have had to stay in a hotel a night and hopefully they would have gotten us out the next day ok. This was a personal trip; but, for my business travel, I would think twice about flying them.