The card can be purchased in a few places, we got it in the tourist office near to Santa Maria Novella. You get a great guide with it that explains all the included attractions, and shows you where they are on a map. With this, you can plan your visit and work out everything you want to see. We saw enough to more than cover the cost of the card, and the benefits were clear:
- no need for reservations, so you can have a more flexible timetable for your day;
- you can use the reservations entrance (there is usually a 'Firenze Card' sign) which means skipping the queues for tickets. This was useful enough in winter, when it wasn't too busy, I can only imagine that in summer it's a godsend;
- it covers entry to attractions and exhibitions as well - by way of example, inside the Palazzo Vecchio, we were ale to go up to the battlements without paying anything extra.
- it is worth pointing out that the card does not cover the Duomo at all, but what it does cover is a fair bit - the Uffizi, the Accademia, Santa Croce, Palazzo Vecchio, Cappelle Medici, the Galileo Museum - the list goes on!
That's all I can think of currently - in short, I would highly recommend getting a card. :)