My wife, daughter and I recently booked a last minute less than 5 days to Hawaii from Vegas. Allegiant all in (ticket, seat selection and bags) was still less than 1/2 the cost of the next closest airline. We had low expectations and they exceeded them. The flights were on time, the staff was friendly and we got there for a low rate.
Downside: the plane on the way there was dirty on the way back was not as noticeable.
Tip: plan to bring some food and drinks. Although the buy on board options were reasonably priced.
Tip: if you buy your ticket online. Spend the money to select your seats online. Being 6'2' I expected little legroom. But after I selected our seats online the leg room was comparable (maybe a little smaller) to united economy plus. Overall it was as comfortable as a 6 hour flight can be.
Overall, I would fly them again.