iPhone 3GS + Magellan Nav app = finally works!
I've used several iPhone nav apps:
NavMi: $4 3G-downloaded-on-the-fly maps for Germany and Luxembourg. The Germans have great 3G, so it worked great AND had voice turn-by-turn.
Magellan Roadmate: $79 fully onboard US maps. No 3G needed. Uses up 3Gigs of space, but works very well. Good turn-by-turn and voice.
Motion-X Drive: another 3G-downloaded nav app for a few bucks. works fine.
skobbler: $5 app that gives you onboard maps of Europe. I'll try this on my next trip.
With regard to heat and battery drain - these are solvable problems. Just do some digging on other threads.
My remaining problem with iPhone nav remains the small screen and glare in bright sunlight. Other than that - I'm set. I connect to in-ear headphones through my Cardo Scala G4 and have phone, music, nav all in one box - all working together. I'm saving my Zumo money for some massive aux lights!
'05 BMW 1200GS
'09 Honda CRF230
'10 Yamaha TW200