I flew Spirit once- Chicago to Vegas and back again.
It is a true budget airline and you can expect all sorts of add on fees for luggage, drinks, etc. I ended up joining the Spirit Club or whatever they call it in order to get the reduced fee on our carry on luggage. As a result, I get continual emails about discount fares and promotions. I have not been able to take advantage of any of them mostly because the closest Spirit location for us is in Chicago which involves a drive from Iowa.
I ended up in a far back row where I am convinced I was in the smallest seating area I have ever been in.
All that said, they were on time and hey- I was going to Vegas, so I can put up with a lot.
Paul