I had a mid ranged sigma wireless computer bc1609 for a year. Worked great until I ran into the problem of headlight interference. Dealbreaker for me.

I then went to the Garmin Edge 800 bundle, (Ant+ has no electrical interference from lights).

All was well but I realized I could sell the edge and with that money buy a quadlock, Bluetooth HRM with money to spare.

That's where I've been for the past couple of years. No regrets at all. No worries about my iPhone 6 surviving a crash in the quadlock case. The newer iPhones have built in barometric altimeters and Cyclemeter app will recognize + display those metrics.

The only thing I miss is the onboard thermometer on the Garmin.

I don't carry additional battery packs unless I'm touring. I can go for a 4-5 hour ride with screen on, running cyclemeter, playing music via bluetooth and come home with 30-40% battery left on an iPhone 6 (not plus model)

I can't speak for the iPhone6+ battery life. They do have much larger internal batteries, but are running many more pixels if you leave the screen on.