I'm not a member, but they have great pricing if you are. My daughter flys back and forth from Chicago to Ohare on Spirit and it works great for her.
You don't get anything on the flight (no food or drink service, no pillows or blanket and very small seats on MD-80 aircrafts), I think you can pay (VERY COSTLY) for soda or water.
[B]The seats are very small and there is positively no leg room.[/B] You have to pay extra to pick your seat and you pay for every bag after your 1 carry on. If you only have carry on and you don't care where you sit or who you sit by, it works.
I am taking it to Fort Lauderdale to visit her in a few weeks and my son is going down for Spring Break -- both on spirit air. It's no frills, that's for sure, but it's cheap!
I just joined to get our airfare to Disney cheap. Paid 39.95 to join and was able to get airfare for 5 people to Orlando for about $800. The person who is the member has to be traveling and booking all the tickets for everyone in the group to get the special pricing. So especially if you're traveling with a group - might be worth it.