I am really happy Hadley put together a nice concise survey of the different OO systems in R! One of the really great things about having these multiple object systems is that they really do lend themselves to multiple styles of problem solving and different domains. S3 and R6 are both really different, but they both can be used really expressively and to build large systems with "proper" software engineering methodologies. If you haven't, check out his other books too... I find them really easy to read but chock full of information.