In reply to steelbru:
I should add that you can't see the grid ref changing as you walk with the 910, you effectively have to say you want to store a waypoint and it will show you the waypoint info on the screen, if you move a bit then you need to cancel back out and repeat that you want to save a waypoint. Sa a wee bit of a pfaff, but ok for occasional use.
The 610 is a lot easier, you just go to the Where Am I screen and the grid ref changes as you move about.
Not sure how the 310 works