I am now using the iPhone 6 Plus mounted on my bike stem. Its really great. I use the GPS and map features, usually via Strava. Its not too big. I bought it because I can't wear reading glasses around while riding my bike, so need the bigger text font, etc. I am using the Topeak 'Ridecase' quick-release mount:
TopeakŪ Cycling Accessories ? Products - 3_RideCase, works w/iPhone 6 Plus/6s Plus, Black
It works great. if you want, you can get a waterproof case from toPeak. I prefer teh regular one; and will shield it from rain by removing it.
As for battery life, I keep a large remote battery attached to keep it charged. I keep the battery in a tube-top bag mounted directly adjacent to the stem. It will run the phone for a good 24 hours, even with the screen on, and the phone running bluetooth connected to cadence, speed and heart-rate sensors. (Not to mention, often playing music.) Tubetop bag is the Timbuk2 2013 Goody Box which you can get on Amazon. (They have later models. Some of them will fit your phone but for supporting a phone, I don't like it nearly as much as the ToPeak mount):
Bike Bags & Cycle Gear | Bicycle Accessories & Carriers | Timbuk2 Bags
Battery I am using a Intocircuit Power Castle 13000mAh External Battery Pack Power Bank Charger. Not sure if this is made anymore; but there are plenty of large external battery packs available.
FYI - as far as breaking the phone, I've had a few spectacular crashes and have not broken the phone. Maybe its luck - your mileage may vary. In one crash, my phone came loose and slid down the pavement about 15'. The Topeak case protected it. Either way, I always keep a hard plastic screen protector on it.