I like Spirit and Frontier. I recently priced a direct Louisville to Las Vegas flight on Spirit for April. It was $255 including a checked bag, a personal item, seat selection, priority boarding and 1st class no security line wait on the return from Vegas. No other airline even offered a direct flight and even with the layovers the other options were in the $400+ to $600+ range.
I flew into Vegas on Delta from St. Louis back in December. While the fare was competitive, I booked basic economy, it had a layover in Salt Lake City, I didn't get to choose my own seat nor could I pay extra to do so and I boarded in the very last group. I got assigned a middle seat on both legs of the trip. The layover plus deplaning and boarding another plane added about three hours of travel time to the journey.
I returned from that trip via Southwest a day later than I was supposed to because of two canceled flights and when I landed I discovered my luggage didn't make the trip with me.
Yeah, I am a Spirit and Frontier fan. For those of us that fly out of smaller markets, they are often our best and a lot of times our only options. In fact, without them I would have never started coming to Vegas again. I am simply not gonna cough up $600 to fly to Vegas on Southwest.
Also, keep in mind United no longer includes luggage with their base fares on most domestic flights.