I have both my iPhone and a Garmin mounted on the handlebars. Details:

1) It's good to have both. I have Sena in my helmet and the iPhone is great for music as well as in-town directions. It gets a little wonky when it loses Internet, but the Garmin always has the maps.

2) I wired in power to each device. Never run outta power on a trip. When I arrive at my destination my iPhone is fully charged. Damn nice!

3) I researched the 'motorcycle-specific' Garmin units and found the most significant features that made them motorcycle specific are a brighter display, weather proofing, and bluetooth input. Oh and a significantly higher price. So I opted to gamble and purchased a nice model 2597 for $128 on Amazon, figuring the worst that could happen would be that it die in some rainstorm. Well I ride in the pacific northwest so its seen a lot of rainstorms and it still works fine. Saved $300 or so there.

Anyway, my $0.02 worth.

Pete