We took SPIRIT for the first time from MSP to LAS last August. Planes were very clean and attendants very nice. We departed and arrived on time or a few minutes early. We didn't pay for seat assignments and were assigned together. It was about a 3 hour flight. All good, including the price.
That said, I will NEVER fly them again. Their seats are the most uncomfortable things I have ever had to sit in. Plus there is very little space between rows. I am only 5-10 pounds overweight (BMI slightly over 25), am 5'5"" tall and have proportionately short legs. My knees were pressed uptight against the seat in front of me. (On other airlines, I usually have between 4 and 6 inches of space between my knees and the seat in front of me). I have no idea how any normal sized guy could stand those seats for more than 10 minutes or so. My DH was so stiff and got off with a backache.
If all goes well, SPIRIT is a low priced alternative. If anything goes wrong, just know you are totally on your own. Their customer service is NON-EXISTENT! They do not respond to emails and IMO, their customer service people (if you can even get to talk to one) are less than helpful.
Above is my opinion based on one experience. Others may have a different one.