I use my iPhone sometimes but have found that when i really need it, I seem to be out of cell phone coverage. But I did buy a universal RAM mount for it and frequently use it to hold the iphone even if I'm not using it as a GPS at the time. I have an Electra Glide and it vibrates quite a bit at idle, and I've never had a problem with the iPhone vibrating too much or falling off. Bumps don't seem to bother it either. I use the Garmin GPS app and listen to the turn-by-turn directions through my Scala G4 headset mounted in my helmet.
BUT, what I've found to be more reliable is my cheap 5-year old GPS Garmin Nuvi that speaks the directions to me. I don't like to have to look down too often at the GPS so I bought a cord for a few dollars that connects the GPS to my stereo on my Ultra. And I bought a RAM mount that specifically is made for that model to mount it to the handle bars.
I used to use the Harley Davidson pouch for my iPhone, but the iPhone sure uses a lot of battery when the GPS is active, and the pouch didn't have a hole in the spot to plug the cord in to recharge it. I ended up cutting one with a razor blade, that works OK, but now that I have the RAM mounts I don't even use the pouch anymore.
2012 FLHTK, Solid Black