GQview is an image browser that features single click access to view images and move around the directory tree. Its development seems dead, but there is a fork called Geequie which seems actively developed instead.
Waiting for the Debian package of Geequie, I still use GQview and I wrote a small Python script to edit Exif UserComment field. You can hook the script to the external editor commands of GQview, so you can launch it with a simple Ctrl+3.
The program allows multi line comments, uses UTF-8 encoding and allow you to edit and existing comment or add a new one. You can navigate in the directory tree and quickly add comments using just the jeyboard.
The exif-comment-edit script uses the exiv2(1) tool to actually read and write the comment, on a Debian system you must install the exiv2 and python-gtk2 packages.
The script is released under the GPL license, and you can download it from SourceForge.