I booked a trip on Frontier from San Diego to St. Louis through Kayak. This was the cheapest flight available which I assumed was due to a change of planes in Denver. However, after receiving my confirmation i discovered I did not have a seat assignment and needed to call Frontier directly. I called immediately and was told I could only get a seat assignment at the airport because of the discounted ticket. If I wanted a seat assignment early, I would need to pay $40 per leg.
On the day of the flight we arrived at the airport with plenty of time. The only seats available were middle seats for all 4 legs of our trip.
I don't know if this seat selection policy is standard for all Kayak bookings or just Frontier but the $100 saved up front was not worth it considering also the time spent in layovers.