Pros of reference semantics: flexibility and dynamic binding (you get dynamic binding in C++ only when you pass by ptr or pass by ref, not when you pass by value).
Pros of value semantics: speed. "Speed" seems like an odd benefit to for a feature that requires an object (vs a ptr) to be copied, but the fact of the matter is that one usually accesses an object more than one copies the object, so the cost of the occasional copies is (usually) more than offset by the benefit of having an actual object rather than a ptr to an object.
There are three cases when you have an actual object as opposed to a pointer to an object: local vars, global/static vars, and fully contained member objects in a class. The most important of these is the last ("composition").
More info about copy-vs-reference semantics is given in the next FAQs. Please read them all to get a balanced perspective. The first few have intentionally been slanted toward value semantics, so if you only read the first few of the following FAQs, you'll get a warped perspective.
Assignment has other issues (e.g., shallow vs deep copy) which are not covered here.