I travelled through Estonia, Latvia and Lithuania this summer with a GPS V and only the basemap. Basically it sucks. The roads featured are only the very major links between the most populated places in the country. If you're thinking of using the GPS V for navigation purposes then forget it.
Instead buy a good paper map (Insight, Michelin, etc.) for working out your route. Then use the GPS V purely to find yourself on the paper map.
For most of my trip I was nowhere near the roads featured on the basemap. However the unit was invaluable in pinpointing where I was in the country and what (fairly big) towns were close by. There were times I'd plumb in my destination for that evening and then hit the back roads, using the compass pointer to help me choose which roads were going in roughly the right direction.
From what I've heard the World Map doesn't fill in much detail in your European basemap. But it does effectively extend the basemap beyond the edge of the one burned into your unit. If you were going RTW it might be a good idea, but I'd still be buying paper maps to do my route planning.
Hope that helps.