The C++ API gives you the objected oriented approach to your API development. It's far more easy to comprehend- and use than the C API.
But deep down it's based on the Notes C API, and you will therefore have most of the can and the can't from the Notes C API. Please refer to these subjects for the overviews.
One important note for OO freaks! You can't create derived classes directly off the Notes base classes. In other words, you can't inherit these classes. This makes some OO-programmers howl about "The Notes C++ API is not a true OO class hierarchy". This is off course correct, but then again, it does solve a bunch of tasks :-)