I agree fully - reading through these comments is frustrating, as the languages mentioned can hardly be called Object Oriented. Learning Smalltalk is something everyone who really wants to understand OOP should do. C++, Java and the like really should be called Class Oriented languages, as demonstrated by the often very deep class hierarchies. It's not about the classes, it's about the communication between objects. Objective-C is far closer to Smalltalk than most of the languages mentioned in these comments.

The downside to learning Smalltalk - the realization that a 30 year old environment is more advanced than whatever modern tools you will need to return to to earn a living.