Is Spirit Airlines's $9 Fare Club worth it?

"worth the investment" is highly subjective. For some people it may be, for others not. For me e.g. it wouldn't be as I never even come close to one of their destinations, let alone flying with them :)
As to what it offers, the FAQ is a tad vague but lists general categories:

What are the benefits of the $9 Fare Club?

The $9 Fare Club offers all of our member exclusive travel deals, not only in the U.S. but all over the Americas. All the way from Lima, Peru in the South to Plattsburgh, New York in the North. Not a bad deal, eh? Benefits include:

· Access to Member Only fares sometimes as low as $9!

· Exclusive offers on hotels, rental cars and other travel necessities.

· Savings of $10 per bag when booking online. (Up to $120 per customer in bag fees, based on roundtrip travel with a carry-on bag and 5 checked bags.)

So in theory you could save double the cost of the "club" just by your baggage fees, depending on how often and with how many bags you travel with them.

After asking the question - and not receiving any answers within the hour - I kept googling around and came up with this: http://www.elliott.org/blog/is-spirit-airlines-fare-club-worth-the-money-can-i-get-my-money-back-if-it-isnt/

So I'm not sure I will be buying into the club.

It depends on a lot of things: where you go, when you want to go, how flexible you are...

The club is better as a convenience purchase than an advance plan. I think the best way to deal with Spirit's $9 club is to wait to join until there's an airfare you want, and can't get without the membership. Sometimes the money you'll save (compared to their non-club fare) will be a significant chunk of the membership fee, so that the fee will "pay for itself" if you use it just once more, and not be a significant loss if you don't.

Sprit has a free discount option: booking in person at the airport. The discount often results in a similar price and is sometimes even cheaper than the $9 fare club. No bag discount though.

No need to pay for this discount, but you will need to make an extra trip to the airport to book, so this is for those with easy access to one of their airport locations.