We always buy our tickets at the counter in the airport prior to our trip as it saves $17-18.
Check in online or at a kiosk in the airport, if you check in at the counter they charge you $10.
Make your "personal item" be your only bag (read online for the dimensions)- this will NOT be considered your "carry on," therefore it will be FREE. We use compression bags and I had no difficulty packing for a 7 day trip with only my "personal item" sized bag. You WILL pay for any other bags you bring on.
If you do choose to bring a "carry on" sized bag or check a bag, pay for it upfront online, you'll get nailed with greater fees at the check-in counter.
The leg room is comparable to any airline and their seats do not recline. Unless, you pay for a large front seat (usually in the first 3 rows). Also, rows 4-7 have more leg room. I've been super lucky with them lately and randomly getting assigned to those larger seats!
You have to pay for drinks and snacks and they do not accept cash.
Remember, you are paying for the Greyhound of the skies...don't fly them expecting Delta! You wouldn't walk into McDonalds expecting filet mignon :)
Overall, I've had 14 trips on Spirit and have never had a poor experience. I've had only 1-2 delayed flights and nothing more than 60 minutes.
Spirit is my go to airline.