How useful is a smartphone? Do you mount it on your bars (rough roads cause some large shocks)?

• Two days ago, I got my first smartphone (Samsung Note 4 - the electronic stylus won me over). I've always carried my cell phone (in a plastic bag in a jersey pocket).
• I also have a Garmin 705 that I bought 5 years ago (six weeks after I started riding - I felt I needed heart and cadence info).
• I plan to keep it in my back pocket unless I get a call (I always stop before I answer any call) or I get lost (rare, but I have always found the Garmin very difficult to use when that happened - it's great to follow a pre-loaded course, though).
• Two friends use their iPhones audio for turn-by-turn instructions - they both also have good Garmin devices, but they prefer the audio. They keep the iPhone in the back. I assume the similar audio is possible with an Android system, but I haven't even looked into it.

What's your experience? Thanks, as always...