One thing I really like about the struct module's format string is the ability to easily specify the endian-ness of the data. How does newstruct (or the underlying ctypes) handle endian-ness (if at all)?
Struct needs it because it is interpreting data. unpack needs to know the endianness to correctly interpret numeric types, and pack needs to know the endianness to know how to serialize.
FYI: the C way of doing this is byteswapping.