By far if you are going to be spending a 4 or more days in Florence, this card is a great deal.
For 60€ you can get a yearly membership to the Uffizi which gives you unlimited access to the Galleria UffiziI and several other museums in Florence, including La Accademia where you can see Micheangelo's David. The year is a calendar year (Jan-Dec) not based on the date of purchase. For two adults and two children the membership is 100€, youth up to age 26 (I'm not sure why after 18 you are not an adult) the cost is only 40 €
If you consider that the admission to the Uffizi is 10.50 € (16€ with reservation), add 6.50 € for the Accademia (10€ with reservation), same for the Medici Chapels, the Pitti Palace, etc ...
Reality that you need more than one day to really explore the Uffizi, and the money goes back into the gallery for restorations and up keeping.
If you are in Florence with limited time the Firenze Card for 50€ may be a good option, it is valid for a 72 hour period from the time it gets activated with your first visit to a museum. However you can only use the card once at each museum.
I intend to get every Euro worth out of mine, I started today ...