Andreas,

Having checked out the Exiv2 road map
(http://dev.exiv2.org/projects/roadmap/exiv2) I notice that you're
considering changing the key from a string to a name.

Could I suggest that you perhaps provide support for both types of
keys (continue supporting string based keys), the reason being that we
make use of quite a few custom xmp fields which would make it
difficult for us to upgrade in the future if all fields were a
constant set of variables.

Also, I'm sure it'd be beneficial to keep some backward compatibility
between versions (and also provide a mechanism for setting/retrieving
custom fields)

Thanks,
Alan.