We were there in the spring, and rented a car and drove to the small towns we visited, and very glad we rented. It all depends on what you want to do, if you want to do, but having a car made it much easier for us to get around without much planning then we would have been able to do with the train. A couple hundred pictures over at
www.cordelli.info/italy if you are yearning for some.
If I were visiting the coast I would probably try to stay there, just because it's so cool to see the sunset and the ocean, then grab a bottle of wine, and not have to head back to Florence. We stayed in Florence the entire time and made day trips, which also worked out great, so either way.
Keep in mind they prevent people from driving into and out of Florence at certain times, so plan around it. Our hotel told us to tell them we were tourists and gave us their postcard to get through, and that worked all but one time, in that case we just parked, waited the hour at a cafe, then went in.