I can really recommend Tommaso's apartment - everything during our one week visit went smoothly.
His friend Marco met us at the flat ahead of the indicated check-in time, as we arrived a bit earlier, and that was really helpful as we could get accommodated immediately. The apartment is located on the top floor of the building which means you need to climb a fair amount of stairs, but we never found this a problem (it is a good exercise after all the food!). Everything was cleaned and ready when we arrived, both the check-in and check-out was quick and easy.
The apartment has a kitchen where you can easily prepare meals for 3 persons (basics like olive oil, salt and pepper etc. were available), and the loft with the extra bed is spacious as well. The air-conditioner and washing machine are great additions and worked fine. The bathroom is accessible from the living room (and not the bedroom, as in some other apartments) which means some added privacy.
I have visited Florence several times, but Tommaso pointed us to some nice restaurants I had not visited before (including good sea-food restaurants), and offered to book tables.
The location of the apartment is really good - a stone's throw from the Ponte Vecchio with also a supermarket close by. Being so central, the fact that it is on the top floor means the apartment is really quiet for its location - this was a good bonus for two of us who are light sleepers.
In short, we made a really good decision staying here.