I've been a "lurker" while researching what I need to know for my upcoming cruise (by the way, thanks everybody!) but felt compelled to register after seeing Spirit mentioned.
I would advise being really skeptical about Spirit Air. The cost may be attractive, but the experience isn't.
They keep costs low because they charge for everything that they consider a "luxury". This includes snacks, drinks, the better seats, and as mentioned, baggage. While most airlines are now charging for bagging, spirit is charging a higher price ($35 vs $25) for less weight (40 lbs vs 50 lbs). You may not think that ten pounds is much, but you will when your bag comes in at 42 pounds and you have to choose between disposing of luggage and paying an extra $25. Also, while other airlines permit free carry-ons, Spirit will charge you for anything you carry on that is larger than a purse. They're very strict about bag weights and bag measurements.
Another issue is that because they don't have as many flights, you have less flexibility if there's a change. Last year, I was flying from Pittsburgh to Myrtle Beach, where we had rented a family beach house from Saturday to Saturday. We booked flights that left on Saturday morning, and returned the following Saturday afternoon. O ER the next few months, we received several emails about flight time changes, usually changing either flight an hour or two back or forth. Then, just a month before our trip, we received an email that our flights would now both be on Fridays. There was no option to change because they only flee two planes a week in that direction. We had to book hotel rooms for the night before our house opened, and ended up losing a day at the house at the end.
Spirit is notorious for bad customer service- just Google them for evidence. Their CEO has stated that he doesn't care about bad reviews, as people will always buy his cheap tickets.
Anyways, off of my soapbox. I promise to post more