We flew with Spirit two weekends ago from Vegas to San Diego for a friend's wedding.
On Spirit you get exactly what you pay for and you only pay for what's important to you. I like Spirit Airlines.
The fact that they sell you credit cards during the flight, NBD to me, we scored last minute bulkhead seats to San Diego for $60 from Vegas where the same flight on Southwest was around $150. For a $90 discount, I could overlook that for sure.
The fact that sometimes the flight can be delayed, also NBD to me, I realize I'm taking a chance by buying tickets at 1/2 to 1/3 the price of other airlines. With that being said, I've flown Spirit 4 times in the past year and my flights have always been on time or slightly early but I understand I'm taking the chance.
The fact that you pay for luggage... again, NBD to me. I've perfected the art of packing a weekend's worth of necessities in a bag the size of a personal item. I prefer not to be charged for luggage as I usually am able to forgo a check in or carry on.
The upside is a brand new fleet of planes, the team members are nice enough, and there is incredible value. Whenever spirit has service to my destination around my ideal travel dates and times I will usually fly with them.
My biggest advice to any future flyer is to upgrade to the larger seats. You'll likely be happier with your choice of carrier.