I used my Edge 305 for navigation when I was stuck for a while in a location where I didn't know my way around. It's much like bt describes and yes, it is cumbersome, but works, for the most part. It doesn't give turn to turn instructions, you pick waypoints on the ride (like just before turns) and the unit will beep whenever you reach the waypoint and shows a name you entered, which could be an abbreviation of the road you want to turn on, etc.
I used this site
http://www.marengo-ltd.com/gps/index.php, which has instructions and a free Google based map you can use to plan your route and name the waypoints. You will need to download gpsbabel (also free) to load the course to the Garmin.
If you try this, I recommend you make a walking route around your neighborhood first. Trying to figure out how this works while on the bike is difficult at best. The instructions at the site above are good for planning and entering the route, but not much help for how to actually follow the route.
Make sure you use the highest resolution of the map to place your way points accurately. If they are off the road, you may not pass close enough to the waypoint to trigger the Garmin. I think this happened to me my first try and I missed half my turns.