I've stayed in Florence several times now and this apartment was the best in terms of the private terrace and the beautiful views of the city. Hard to beat this.
The location is great - in the quieter Oltrano district - and a 10-20 minute walk from all the key attractions in the centre.
This said the Piazza is busy night and day. The bars are open into the early hours and the cleaners are there a few hours later to get it ready for the working day. I wasn't too bothered but some might find it hard when trying to sleep.
Note, as the reviewers have said, the apartment is in the 5th floor and there is no lift. So take this into account with your luggage - the apartment may not suit some with young children or those who find many steps a problem.