I have a garmin Streetpilot C550. I only have road use with it. I figured for $10, what the heck (I already had Bizer's map). I had to uninstall Bizer's before I could install the Garmin Winni.
You end up with two maps: the Garmin City Navigator and the winni.
The day I installed it, it was raining, so no golf and no boating. Call me fair weather
. Living in Alton Bay, I tossed the GPS into my car, got it up and running at my house (a half mile off the lake) and drove down to the bay, pulling into the parking lot of the bay. No indication of the bandstand or no wake area. I was looking out to Sandy Point, and no markers off the Back Bay bridge, the black and red spars. I drove out to Sandy point and no markers, not the flasher and no spars.
I then went home and uninstalled the map and reinstalled Bizer and said a silent thank you prayer to Bizer.
If all you want is an outline of the lake, then spend the ten bucks. If you want some good navigation, get the bizer.
As far as how it knows where to go, I assume, and I do know what happens then, is that when you select the map, it knows to check the internal as well as external sources.
With Bizer, you also have the safe routes from say Wolfeboro to Alton, Center Harbor to Weirs, etc. I also felt the depth contours were far better on Bizer, even with only about a half hour trial of the Garmin Winni map. If you do go with the Garmin, I would select the Road setting, unless you can get "bread crumbs", or your outbound trail, in bike or pedestrian.
Auto will not allow bread crumbs. I set the dock as a favorite, was out in the middle of the broads off Rattlesnake, selected go to dock, and it did its calculation and told me to drive to highlighted route, and then turn left, which was route 11.
IMHO, the Garmin chip isn't worth the ten bucks and time to put it in.
Hope that helps
Dave