I'm living in Turku and do most of my mapping in the city. Currently I'm trying to map cycleways.
I started mapping with a Nokia N810 but got very frustrated with the GPS startup times. Recently I bought a Garmin eTrex Legend and have massively increased my mapping.
Links to sites I have found useful:
OpenStreetMap maps for Garmin devices:
MTB map for Finland: http://www.polkupyoraily.net/osm/
Maps for the rest of the world:
How Garmin devices work: http://www.gpsinformation.org/dale/gpsfix.htm
Make your own maps: http://wiki.openstreetmap.org/wiki/Mkgmap