i always found C++ IOStreams ill-designed: their implementation makes it very difficult to properly define a new type a stream. they also mix io features and formatting features (think about manipulators).
personally, the best stream design and implementation i have ever found lies in the Ada programming language. it is a model in decoupling, a joy to create new type of streams, and output functions always work regardless of the stream used. this is thank to a least common denominator: you output bytes to a stream and that's it. stream functions take care of putting the bytes into the stream, it is not their job to e.g. format an integer into hexadecimal (of course, there is a set of type attributes, equivalent to a class member, defined for handling formatting)
i wish C++ was as simple regarding to streams...
