We used to use Spirit on a regular basis. We never had any problems with lost luggage or being bumped, but we frequently bought exit row seats for the leg room. They now nickle & dime you, making you pay for carry-on luggage, checked luggage, seat assignments, etc.
DISCLAIMER: We don't use them anymore for two main reasons: 1) SW bought Air Tran so we can reschedule without penalty. 2) We have the Delta Amex card and get free checked luggage. Of minor consideration was the fact that Spirit has very little leg room & I was tired of my knees in someone else's back.
Good luck!