I flew Spirit for the first time this weekend and was very pleasantly surprised. It seems like almost all of the people complaining are angry about their fees. That problem could easily be solved by reading. Yes, they charge for everything, but it's posted on their website in about 500 places plus you have to agree to the terms to book, so I really don't understand how anyone, unless they're illiterate, could be surprised.
Anyway, the plane was clean and the staff was polite. I bought a hot tea and a humus snack for $6, which I thought was reasonable. For what I was looking for (didn't need to take bags on this trip) it was a perfect choice.