I have TomTom on the Iphone 3Gs. A regular TomTom One car GPS.
The TomTom iphone is a battery killer. I always have mine plugged in. Recently in Dallas on a biz trip it would not refresh fast enough at 70 mph ( I was not driving ) and we missed a few exits. Bottom line it's about useless unless you keep it facing the sky thru the windshield so you need their $100 mount. <img src="/forums/images/graemlins/zombie.gif" alt="" /> Now with that said it was the first generation of the software, I've upgraded since then but have not tested. My phone became hot to the touch after a hour of TomTom use. I held it over the AC vent !
On the flip side the TomTom one works flawlessly but like you mentioned the whole multi-device thing is a pain especially for travel.
I've tried many TOPO type maps for the Iphone
www.topomapsapp.com and the Yosemite map by EarthRover.org
They take a while to lock on without cell coverage but work.
However they lack more advance features a full fledged unit like my Garmin 60Cx unit. As far as confidence factor I have way more confidence in the Garmin than any Iphone app.
THE MAIN PROBLEM WITH ALL CURRENT SMART PHONES IS THEY DON'T SUPPORT WAAS. Sorry for the yelling but I was PO'ed when I found that out. Without WAAS your pretty much hosed without cell reception. Even with cell tower reception you need a few towers to do triangularization math. Most places off the beaten path will only get one tower so that goes out the window fast.
For now the Iphone is a neat toy and a nice backup. I don't trust it enough for serious use but is nice while traveling.
The built in google map can get you where you need to go without paying for the TomTom software in the city. Plus my Garmin will lock on inside my house !
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