West Coast Motors (http://www.westcoastmotor...) provides service throughout the area highly dependant on where you want to go. It serves the main routes and even goes to Inveraray to meet up with Citylink bus services to Campbeltown and Oban. There is also a service to Portavadie where there is a Cal-Mac ferry service to Tarbert. There are also bus services to the Isle of Bute, which stay on the Colintraive - Rhubodach ferry for 5 minutes of the journey. Expensive and irregular, it can be a pleasure if you have a friendly bus driver. There are plenty of overpriced taxis vying for your trade in Dunoon and numbers are always available.
A best bet for a visit is to take a car, as this is necescary to get to the best walking spots and off the beaten track.