As I have Pentax camera, I wanted to be able to extract information from maker notes it stores to EXIF. As only tool which seems to be capable of this is ExifTool and it is a Perl beast which I really don't want to use from Python, I need to add support for it to some existing library.
I found two which could be used - libexif and Exiv2. Because of availability of Python bindings I choose Exiv2. The reason why I also prefer this is upcoming XMP support for which I was looking recently. The only disadvantage I can see is that libexif is more widely used by applications. Why the hell couldn't be just one library doing this?
Anyway if you're interested in Pentax maker note support for Exiv2, you can find it in their bug tracker and it will hopefully appear soon in their svn ;-).
