I've flown Allegiant to Vegas four times now and my experience has been mixed. On the positive side, the fares can't be beat and flying out of Gateway is as easy as pie.
On my first trip, we had to turn back to Mesa as soon as we took off because of a smoke detector going off in the bathroom. The fire trucks were waiting for us with lights flashing and everything. At this point, I was a little rattled and contemplated just driving up to Vegas. We decided to stick it out, they found another plane and we were on our way in a little over an hour. Made it there incident free.
Second trip was a breeze. No complaints.
Third trip had a ton of turbulence. Not the airline's fault I suppose. I'm not a pilot, but I do question their decision to fly in those sort of conditions. The plane was rocking in every direction. Scariest flight of my life. Luckily it was just a short hop.
Fourth trip went as well as it could have too.
My only complaints with Allegiant stem from the way they do business. I have a bag that I carry-on to every other airline without issue. When I checked-in at the gate with Allegiant they measured it and it was like 1/8 an inch too big. They let me "slide" and told me I'd have to check it in next time. I think they may need to recalibrate their ruler though. Another complaint is the limited ticket counter hours. I get why they do it (to make people buy online), but it seems backwards to charge less to actually use the services of a person. Also seems they only staff the ticket counter with just one person to further dissuade you from buying at the airport.
Lastly, their fleet is one of the oldest in the industry. I hope the maintenance issue I had on my first trip is just an isolated incident rather than a regular occurrence. Because of this concern, I'll probably only use them for short trips rather than going across the country, and especially not Hawaii since there's nowhere to make an emergency landing.