Some users have reported a problem that makes the Inkscape extension unusable on Windows. When opening Sozi, an error dialog shows the following message:
glib.GError: Couldn't recognize the image file format for file 'sozi.py.svg'
Sozi 12.05 fixes this bug. See the instructions to download and install it.
If Sozi 12.04 works correctly for you, you do not need to upgrade.