We have just returned from Italy and a visit to Florence. We purchased Firenze cards and found them to have great value. There, as always, are a couple of caveats that come along with that.
First, the value of a Firenze card is not really economic, at least not for us. It is one of convenience and flexibility. I think the sights we saw with the card would have cost 55 euros. The Firenze card allow you to show up at any time of day and have a place in the reserved line. No commitment to a specific time. Complete flexibility and you definitely need a timed reservation to see the Uffizi and Academia. To me, that was worth 17 euros, but you need to make your own decision. Also, because you have pre-committed your money, you may stop into places that you normally wouldn't have and find something quite interesting. For us it was the Galileo Museum. Very interesting. A nice break from the art overload that can happen in Florence. But I don't know if we would have ever gone in if it wasn't covered on the Firenze card.
Second, in both Venice and Florence sights, "skip the line" doesn't really mean you don't have to stand in line. It means you get to stand in the short, fast moving line instead of the long non-moving one. It took about 10 minutes to get into the Uffizi at 8:30 AM, about 15 minutes at the Academia in the late afternoon, and about 30 minutes at St Mark's in Venice (our longest wait but the regular line was more than 2 hrs.).
Last, I don't think you have to do the Duomo sights all in one day. Once you have your ticket, you just present it at each location and they check off which one you are visiting. I think you'l be able to climb the dome one morning and the tower the next day. We did not climb either but went to the Duomo Museum one day and the Baptistery the next. As long as you are within your 3 day limitation on the Firenze card it's OK.