We flew Allegiant Air this past January, round trip Lansing to Orlando/Sanford.
The planes were fine and we loved it being non-stop from Lansing instead of either having to get on another plane in Detroit or driving to Detroit to get on.
Small problem on the return flight, we were on the plane and ready to go when they discovered a small oil leak. They thought it would be okay, but still decided to put us on another plane so that took awhile to do. But I would rather be "safe than sorry" so it was no big deal, just put us an hour late getting home.
I would definitely fly Allegiant Air again.