In a nutshell - the apartment is great in any aspect you can think of and Candida is a lovely host.
I stayed in the apartment for 3 months with two housemates, each one in a different bedroom. All of the rooms are very comfortable and spacious.
The apartment has all the things you might need - washing machine, dish washer, wifi connection, a big fridge, a microwave, two toilets, a big flat screen tv and so on. The kitchen is well equipped as well (today even more, because we bought more things from time to time during our stay).
Location: When I booked the apartment I didn't really realize how good its location is - It is really GREAT. 5 minutes walk from the Duomo and 4 minute from Santa Maria Novella train station (and I know for sure because I walked there almost every day).
Design: The apartment is not ugly in any way, but it's doesn't have an outstanding interior design either. I think it's the only bad thing there is to say about the apartment (maybe except for the noise from the street, but this problem can't really be avoided in this area of Florence and in my opinion it's not so terrible anyway).
Another thing to say about the apartment is that it is on the top floor of the building but it has a well functioning elevator.
Regarding Candida: Candida is really a great host - she is very thoughtful and she was very helpful in cases we needed to consult someone.
Price: The apartment's price is really outstanding for what it offers. During my stay in Florence I participated in a student exchange program and I got to talk to many students about their apartments and rents and I can clearly say that this is a catch.
I highly recomend this apartment for all of the reasons mentioned above.