We have an eTrex Vista. It's small, it has plenty of memory but it is expensive. The model we bought in the US has the maps from North and South America, so we also got the WorldMap CD to load the maps from Africa and Asia (you can easily fit half of Africa into the GPS).
In many places in Africa road signs are inexistant so the GPS was extremely useful there - a very valuable complement to the map. In South-America, it wasn't really necessary - just nice to have - except for the two points below...
One of the uses we liked was to be able to tell if we would be able to make it to destination before or after nightfall. In fact, many times, we relied on it to get as much sunset light as we could for photo shoots before arriving in town at night.
The second use that was really invaluable to us was to find our way back in cities: mark your hotel, go out in town with your bike and when it's time to ride back home, put the GPS in traceback mode and just follow the line that you left. Very very nice - especially at night.
For the rest, it was as I wrote a nice complement to the map. Even though the WorldMaps don't give any detail within cities, seeing our trace on the screen made it much simpler, for instance, to find the hotel that we had spotted in the guidebook.
We almost never got lost during the trip. Whether or not it is a good thing, it's your call. For a true sense of adventure, getting lost is something that some people might be looking for (and running out of gas, even more!)
Don't forget to buy a 12v adapter. These things use the small AA batteries like crazy. For the GPS mount, I recommend the RAM from CycoActive. It is much cheaper than the Touratech model, it doesn't obstruct the buttons as much, and the additional height and better orientation makes the GPS easier to read:
Pierre (& Merritt too)
[This message has been edited by pierresas (edited 14 November 2004).]