In November 2017, I managed to visit all the 14 locations of this WHS: twelve villas and two gardens. Only some of the villas have regular opening hours, a few others are accessible under certain conditions (eg a hotel or a winery) or by prior arrangement. When you travel by car, all locations are easily accessible, public transport seems to be quite time-consuming for the remote sites. However, the best sites are in or around Florence: the Villa della Petraia, the garden of the Villa di Castello and the Giardino di Boboli provide a good impression of this WHS. If you want to visit more, I would recommend Villa di Poggio a Caiano and (to a lesser extent) Giardino di Pratolino and Villa di Cerreto Guidi.

North of Florence:

Villa di Cafaggiolo: not open to the public; 30 km north of Florence, currently under scaffolding.
Villa del Trebbio: no regular opening times; on a bumpy dirt road not far from Villa di Cafaggiolo. The villa is hidden behind walls and high trees. Today it is owned by a wine and olive oil producer, so a visit by special appointment may be possible.
Giardino di Pratolino: regular opening times; 15 km north of Florence. The original Medici villa was demolished. The park is a landscape garden with several fishponds, statues, aquaducts. Most impressive is the ‘Colosso dell’Appeninno’ (photo), but it was also the only pond filled with water. The poor condition of the empty pools and some sculptures suggest that the fountains and water features do not work in summer as well. Quite nice, but also a little disappointing.

In Florence:

Villa di Careggi: currently closed for renovation works, the park was also closed; previously in use as an administrative building by the neighboring hospital, but I’ve no information on the future use after restoration.
Villa della Petraia: located in Castello, regular opening times, visit with a guided tour, free of charge. La Petraia was my favourite, the characteristic architectural elements are the massive tower and the beautiful courtyard entirely covered with wall paintings. The furniture and internal decortation are mainly from a later period (18th and 19th century), most remarkable are the fourteen lunettes of the Medicean villas by Giusto Utens.
Villa di Castello: located in Castello, just a short walk from La Petraia; the villa is not open to the public, but the garden has regular opening times; in detail described in the review by Els, nothing to add.
Giardino di Boboli: behind Palazzo Pitti; huge gardens with plenty of sculptures and all the typical elements of a Renaissance garden, I liked best the Grotta di Buontalenti and the Fontana del Bacchino. If you want to visit the gardens only, I would recommend to use the side entrance a few hundred metres after the Palazzo Pitti.
Villa del Poggio Imperiale: located in Arcetri, about one kilometer uphill from the Boboli Gardens. Today it is used as a school building, it houses also a small museum with limited opening hours. The museum was closed when we arrived, but we asked the concierge and were allowed to have a stroll around the corridors and the the vestibule.
Villa Medici di Fiesole: not open to the public, not remarkable from the outside. Several years ago, during a business meeting, I was invited to a diner at Villa Fiesole. I remember the nice garden and the amazing view of Florence.

West of Florence: 

Villa di Artimino: today a four-star-hotel and restaurant, we visited from the outside only.
Villa di Poggio a Caiano: 20 km from Florence; one of the most beautiful Medici villas, with many typical Rennaissance elements, regularly open, but most of the interior is from later periods.
Villa di Cerreto Guidi: houses a hunting museum with regular opeining hours, but the museum has no relation to the Medici and is not very interresting.
Villa La Magia: currently under scaffolding, guided visits only on appointment.
Palazzo di Serravezza: the most western of the Medici villas, located between Lucca and La Spezia. The villa looks more like a castle. Today it is used for temporary exhibitions, but it was closed, we could only visit the courtyard.