The basic usage is:
nntpfs [username[:password@]]server[:port] mountpoint

username, password, port are optional
when successfully started, the command will display debug messages and will continue running in the current terminal until the directory is unmounted. 

for unmount, use the command "fusermount -u mountpoint", the fusermount package should be installed. Alternatively the umount command can be used, but root access may be needed.

In general it is safe to stop the application using Ctrl-C, however the mountpoint will appear as invalid until unmounted.

Cached data will be saved in a newly-created .nntpfs directory in the user directory. As stale data is not currently dealt with, the only work-around at this moment is manually deleting the cache.

If the nntpfs command was successfully started and it is running
(look for "Connect succeeded." message and "FUSE:loop"), then in the mounted directory, a hierarchy of groups should be available.

In addition to directories corresponding to groups, there are several special directories starting with a dot (ok this may not have been very smart)
.files
.sorted
.byauthor
.bydate (not working yet)

listing the contents of any of these directories, or accessing any file inside will result in loading the group contents from the server.

This is done in order to allow navigation within the group tree without loading all group contents on the path. After that Individual messages can be accessed as files. Multipart messages are dealt-with and they appear as a single file.

Warning: listing group contents may be a time and bandwidth consuming operation, the number of posts in a group is found as the size of the .files folder. Keep in mind that the group contents are kept in memory, although in a compacted form.

The .files directory contains a list of all messages (this may be quite large).
The .sorted directory contains files grouped in directories by using a heuristic.
Everything that could not be categorized is found in the .other directory.
The .byauthor directory contains messages grouped by poster.
The .bydate directory will contain messages grouped by date.

Filenames may sometimes contain special characters, but in general they seem to be well tolerated by programs like Midnight Commander.

Warning: using KDE/Gnome programs with a preview function may result in accessing all files in a directory.

Cool use of this tool: 
You can preview files, unpack archives directly out of the newsgroup, without the need of storing the file locally.