for me, the availability of maps on the device itself is important. I am very often abroad, and downloading maps is far too expensive. So, ovi maps is good, but is alas without voice guidance. I am using Sygic 10 to my great satisfaction. The maps however are often not detailed enough.
I am keeping an open eye on what is being developed: navit seems to be good, maps can be preloaded, but the app is not yet stable, mappero cs is fun. special mention of QLandkarte M, for geotiff topographical maps. No guidance of course, but the topo maps at hand in the field is great!