dynamic_cast<>()

When is the last time you used dynamic_cast<>()? I used it today... but I was wondering if the person that has to use my code later would understand how to use it or why I used it. I think you could always avoid using it but why should you? My reason to use it today was to avoid a 'fat interface' (24.4.3 of The C++ Programming Language) for a pure virtual base class. C++ has features that the average programmer doesn't know about. Is it ok to use them? If it isn't ok to use them then why have them? Will it ever be ok to use them?