First thing you need to know... this is a budget airline. This isn't Delta or American. So if you are looking for certain things know they come with a certain price. Allegiant did what I needed it to which was get me and my luggage from Austin to Vegas and then back again in one piece and on time.
Allegiant charges extra for everything so expect to either pay extra for stuff or not get it. Things that cost - checked bag, carry on bag (they allow one small item such as a purse or small backpack that fits under the seat free), beer, soft drinks, food and even WATER all cost on the plane. Two things I have never not seen on an airline that I found on Allegiant.... none of the seats recline and there are no air nozzles things on the ceilings above the seats (where you can turn your own air on or off). The plane was somewhat dirty in that it is dated.... carpet peeling in places, signage on seats peeling off, etc.
All the staff we encountered were very nice though.
Again, this is a budget airlines so if you keep that in mind when you get excited that your round trip flight somewhere is only 120$ you will be ok since you will know you will have to pay extra for everything elese.