I have been playing about with a Nokia N810 for a few weeks now and only today did I manage to get the GPS working at which point five minutes later the battery gave out.
So I didn’t really get a chance to play as much as I would like.
Like most GPS devices you do need to leave it for a while watching the sky before it can work out where it is.
Once it did, there I was correct on the map.
Alas there is no included navigation software on the Nokia N810, you need to purchase additional software at €99 for that, and at that kind of price I would choose a dedicated satnav system.
Where I think the GPS will be useful is when you are lost and not quite sure where your destination is.
Charging the N810 up again and I might have another play tomorrow.
take a look at maemomapper (which does navigation) and navit