Read the fine print of the contract they send you.

A lot of jet card companies are buying blocks of time through brokers and are acting as a middle man or middle men if goes from You --> Jet Card --> Broker --> Operator.

So you'll want to know who is actually flying your plane(s) for you and what their safety reputation is.

Other things to look into/ask:
How old are the aircraft that your card buys?
What happens if they provide you an aircraft that is not capable for your mission needs (i.e. you're going skiing but it doesn't have the room to carry your skis?).
What happens if the plane has a problem mid-trip?
What does their guarantee mean, if one is offered?

The main take away is that you want to make sure that the form agreement they send you protects YOU.

I reviewed the contract for a client who was looking for a low dollar jet card from a large jet card provider. He did not know that the card he was buying was for lift on planes that were over twenty years old and that nowhere in the final agreement did it explain what the guarantee that was splashed all over the provider's website meant.

I hope this helps but if you need any further help - feel free to message me on here. I'm a business aviation attorney and frequently represent people/companies in the purchase/sale/lease/finance of jets and fractional interests.