Very small, but functioning airline.
We flew Allegiant Air and had no problems checking in and finding our gate at the airport. The wait wasn't so bad and the garden/outdoor/atrium area that they have (after passing through security) is a nice little place to chill out and wait.
Don't plan to dine here or anything outside of buy water and snacks, as there isn't much going on. The bathroom and waiting areas are clean. Though, there was a bird who had flown in from the garden area... kind of cool though. Maybe not if you do not like birds.
I will say, check with the folks that you are coming to visit, as you may want to fly into Sky Harbor. We didn't and it ended up being an hour drive for them to pick us up. REALLY great friend I have there. :)