Bus travel in Italy

Not actually. Buses, in Italy, are almost always complements to trains. And slower. Don't be put off by train transfers: they are normal, and a mainstay in most itineraries.

Gail, Some towns have better service by bus. Siena is one of them. The bus from Rome is from tiburtina,across the street from the train station, see http://www.sena.it/index_e.htm
The bus is faster than the train, does not require a change, and lets you off in town.

There is bus service from Rome to Siena, but trains are more frequent and only require one change in Chiusi. The Chiusi station is small and the change is easy. From La Spezia to Orvieto, train is your best option despite the multiple changes.