I joined last December and at that time if you were a member you got $9 dollars off instantly on your ticket purchase. For me that paid for the fee after the first round trip flight. Now if you join what you get is the ability to buy the special $9 fares when you get thier emails. No more instant discount. If you don't have the ability to take advantage of some of those deals in my opinion it would not be worth it. If on the other hand you can then it would be worth it.
Just as an aside a couple of things that I learned with spirit is that if you are a frequent flier member your mile expire after 6 months if you do not fly again. Secondly if you happen to want to pay for what they call the Big Comfy seat you cannot find out if it is available until after you purchase your ticket. The same with picking seats you cannot until you pay, then you would be billed the extras fees for the other stuff.
That being said if the fare was right I would still fly on them, we just came back this August.