I would not think that the money spent on a Spirit mileage run would be worth it. Their fares are generally quite cheap, so what you're spending earning miles would far exceed what you could buy a ticket for instead of redeeming those miles for the same award.
Programs: AS MVPG & Board Room, Marriott Silver, Global Entry
I'd join, but wrong cities. I'd probably do it for the same reason you're considering it...just to experience it. It's your money. Do it, no matter the reason....If you want to use NK to go to an airport you've never been to, then GO FOR IT! If you want to use NK because you want to see just how bad they are, GO FOR IT!
If you never fly them again, at least when you trash them, you can say that you have been on one of their flights.
Programs: AA-Hilton-Raddison-BA-Benihana.. ALL PLATINUM LT
I got lucky and scored one of those "runs" to Las Vegas and actually had to go there for other reasons than getting wasted and lose money.
Anyhow, I had no problems what so ever with Spirit besides being late both ways. On the way back I was in brand spanking new plane. Never been in one like that before. New or reconditioned, but it looked super clean compared to usual AA. And on top of that I won 300 hundies and paid off my hotel bill at Hard Rock.