I’ve flown them 3 times out of Logan, to Vegas, Chicago, and Orlando. When they’re on schedule, it’s fine. The worst was going to Chicago, someone on my flight stole my wallet (which could happen anywhere but doesn’t help my view of Spirit). Then when we were in Chicago flying home, our flight got cancelled and that was the only flight of the day with the next one leaving in 24 hours, so that sucked and we had to get on a JetBlue flight last minute instead.
I’ve also always paid for a seat because I don’t want to get stuck in the middle, so the extra charges add up. Plus I hate that for a checked bag the max weight is 40lbs instead of the normal 50lbs. I need the extra weight for my overpacking!