I have been using the iPhone as a dedicated gps on my bikes for a long time now. I have the phone in an otterbox case which makes it resistant to water, dust, etc... I removed the clip from the otterbox holster and mounted this onto the iron aces mount on my Road Glide. I have the different setup for the iPhone on my Ultra.
I also use the Navigon software, it's light years better than any factory or Garmin GPS that I have ever used. Graphics are great, lane assist, search, traffic, weather, etc... With my iPhone in front of me I can also see important calls, texts, check the weather, etc... If I plug it into the aux in port on the radio, I can have the nav prompts come through the radio, use it as an iPod and even listen to Pandora or Sirius or whatever I have an App for. When I get to where I'm going I pop it off and stick it in my pocket.
After using this setup for a couple of years and having used dedicated Garmin setups in the past I would honestly never go back.