It depends on how much you're paying for the flight and how long the flight is. If the fare with baggage fees is at least 30% less than another airline and the flight isn't over two hours, I usually consider the Spirit flight.
I've never had issues with their employees, I've always found the flight crews to be pleasant and helpful.
However, they do add a row or two extra to the planes and an extra seat to the rows (instead of two and three seats to a row, its three and three). You are squished in, which is fine for 90 minutes with a cheap fare, but not on a four hour flight.
The airline has always been timely on the flights I've taken. They've left late on a few flights I've taken, but I've attributed that to the airport, not the airline and have always made most of the time up in the air, arriving less than 10 minutes late.