I'm a owner of a Garmin Forerunner 305. To use it under linux, there are two ways.
Use of my.garmin.com
There is a plugin to install on firefox to be able to retrieve data from the garmin, and send data to the website my.garmin.com . Simply add the following repository in ubuntu/debian and install garminplugin
sudo add-apt-repository ppa:andreas-diesner/garminplugin
sudo apt-get update
sudo apt-get install garminplugin
The main page for this project is http://www.andreas-diesner.de/garminplugin
Local use of data
In the first part, data are all uploaded and stored by garmin. you have no data on your computer, and you can't get them!
In linux, you can store the data from the forerunner with the following program, available in the normal repository, with
garmin_save_runs. The package name is garmin-forerunner-tools . The main website for the project is http://code.google.com/p/garmintools/ . Now you are able to convert these data and end them to different programs