We had no issues on our flight with Spirit. Know going in that there isn't an abudance of leg room. DH is 6'1" and at the time was over 300lbs. He said it wasn't unbearable. But he didn't think he would do a flight longer than the 2 hour flight we did. But for $800 in savings he thought he could "suck it up".
Be an educated consumer. It really irritates me when people say "they don't tell you till you get to the airport" when it's on their website. It isn't hidden. This is the link from their home page:
http://marketing.spirit.com/how-to-fly-spirit-airlines/en/ And their fees are on their website under "Optional Services."
If you need to check a bag, pay for that in advance. Not when you get to the airport. Know that there is a 40lb limit. I packed a luggage scale so I would know when packing on the return trip if we were overweight on the luggage. If you need a carryon bag larger than the "personal bag" that is free - pay in advance. It is considerably more expensive to pay for it at the airport. Each of us (me, DH and 2 kids) each took a backpack for free. From their website:
Personal item (e.g., purse, small backpack, etc.): Dimensions must not exceed 16 x 14 x 12 inches (40 x 35 x 30 cm) including handles and wheels
Remember: You can bring one personal item and bring items like an umbrella, camera, infant diaper bag, assistive devices, outer garments (e.g., coats, hats and wraps), reading material and food for the flight for no additional charge.
Print out your boarding pass at home or at the kiosk at the airport. Don't go to the counter to print it. There really is no reason too.
If you live close to an airport, purchase the tickets at the airport. It is cheaper as there is no "passenger usage fee". When we went to the airport the lady at the counter said "so you know the secret!"
They charge for drinks. If you don't want to pay their prices, you can buy food/drink at the airport after security and bring it onboard. The aisle in front of us last time had a whole meal from Wendy's. Smelled like french fries for 2 hours.
The seat assignment thing is up to you. Last time we purchased it at $12 per person per flight. This time the lady at the counter when we bought our tickets told us we would be seated together since it was purchased together. My coworker didn't purchase in advance and they were seated all together. As long as we are together, I'm good.