I flew Spirit once (R/T). We took the entire extended family of 19 people (I think three might have been lap children) from LGA-DTW for a relative's wedding for under $1,000 total. At that price I was totally happy with the service and would do it again if I had the opportunity.
That was however, before they started charging for carry-on bags. It was a commical site to see all the little kids in my family tugging their roller boards through the airport.
For business travel paid by someone else, I have never taken Spirit and don't expect to (for numerous reasons), but there might be enough leisure travel for them to do allright.