My first time on Allegiant. Flew LAX to BLI (Bellingham) and return on 5-25 and 5-31, both A319. Observations:

I did not pay for any seat. Checked in using the app. Was given 22F window on check-in at T-23 hr on outbound; it was a nearly full flight. 15E middle on the return, checking in at 15 hrs before the flight - there were at least 15 empty seats on the flight, including a couple aisles and windows that it didn't give me. (I really want to know the optimum time to check in to avoid a middle, but I'm not sure)

I also did not pay for baggage, and carried on a backpack that was more or less at the size limit. Size was not checked on either flight, at least not with any measuring device. LAX departure, anyone could have used the overheads, and they boarded groups 2-4 together. BLI departure, there was some segregation: groups 2-3 were identified as "no overhead" by the gate agent, and she checked that each passenger was only bringing 1 item, but once on board, the FA's wouldn't have known what group you were.

Flights were within 15 minutes of schedule, and no problems on board. Seats were hard, but no less comfortable than UA's Recaro seats, and I wouldn't recline on a short flight anyway. The only minor issue is that there is only a tiny magazine pocket, so simply carrying a water bottle leaves you no place to put it. The aisle is surprisingly wide (more than 1 passenger commented on it), but I'd rather they had given an extra 1/4" or 1/2" seat width (or shoulder spacing).

Short review: I might use them again, if it saves >$50, and if its a short enough trip both where I don't care about a middle, and can put my bag at my feet.