Be prepared to add $15-30 each way for "perks". You get a carry on, but that's literally a purse or backpack, if you want to carry on a real carry on piece of luggage to put in the overhead bin it will cost you. You also have to pay extra to reserve any seat before the day of travel, if you have little kids this can be an issue, I don't know what it's like when you get there that day with little kids (I've always just paid it) but I would hope that they wouldn't leave a kid unattended but they do make you pay an extra 10 bucks per seat to reserve them together before hand.
They can still be a decent option, it's just rarely as good as it looks to begin with. I've heard that Allegiant is usually the airline that provides other airlines with the "some airlines charge you these hidden fees" that they use in all their ads.