The use of GPS
For critical navigation do not use GPS unless you are certain of what you are doing! The following are only my observations. For accuracy in conjunction with a map a GPS must be set to the same coordinate system to which the map was drawn. In the UK all the maps I have seen appear to be based on the Ordnance Survey. Therefore the GPS must be set to the OSGB grid and OSGB datum. This involves two settings on my Garmin Etrek. (For maps with a Lat/Long grid the datum should still be OSGB) I have noticed a discrepancy of around 200 yards (Metres) when my GPS was set to the WGS84 (Default) datum.
The GPS is a useful tool for navigating on land, while walking it keeps locked when in my shirt top pocket, except under heavy tree cover when holding up in my hand improves things. It will sometimes lock indoors when there is not too much above. Except when surrounded by lots of tall buildings it works well in the car under the windscreen.
An unknown destination may be reached with it's help by keying in, or transferring from a computer, the coordinates or route. The coordinates may be found by looking at a map or if you know the postcode going to a site like http://www.streetmap.co.uk/ which will give a map and coordinates. Note that a location obtained like this is not precise. A postcode derived location can be perhaps half a mile out! Most GPS units may be used with a computer running an accessory mapping CD, which should make things very easy.
Whatever GPS system you use, it is a very good plan to look at a map before. With a simple hand held device following the arrow requires some idea of the route, occasionally an in car Satnav sends vehicles on impassable tracks.