I don't have a data plan. Can I still use 5800 Navigation Edition? Or it the GPS feature completely useless? Thanks.

Nokia/OVI Maps can load map and POI data from the memory card. So you can, in fact, use the GPS feature without data. Just make sure you turn off the Assisted GPS feature, because that uses data to get faster satellite locks (by downloading current almanac and ephemeris data from local cell towers). Note that the satellite map view seems to use data, so stick to the normal map views.

With A-GPS off it will take longer to get a lock, but it will work directly from the satellites. With standard map views and A-GPS off it should use no data. EDIT: also do only off-line searches.

To get map data just hook your 5800 to your PC via USB and use Nokia Map Loader (although the Nav edition likely has maps pre-loaded).