rhettinger: users frequently need sys.getsizeof. See, for example,

http://stackoverflow.com/questions/1331471/in-memory-size-of-python-stucture
http://stackoverflow.com/questions/449560/how-do-i-determine-the-size-of-an-object-in-python
http://stackoverflow.com/questions/2117255/python-deep-getsizeof-list-with-contents
http://stackoverflow.com/questions/11301295/measure-object-size-accurately-in-python-sys-getsizeof-not-functioning
[and so on]

In what cases, do you think, it generates meaningless results? I find the results for the shared-keys dictionary quite meaningful and natural.

sys.getsizeof is certainly CPython-specific. However, __sizeof__ is not just an implementation detail of sys.getsizeof, just as __len__ is not an implementation detail of len(). Authors of extension types are supposed to implement it if object.__sizeof__ is incorrect.