I've owned pretty much all the GoPro's and honestly, they aren't great cameras. They can do good stuff in the right hands with the right experience level, but they are specifically made for the sports/crash-cam market, not for going out and shooting stuff with friends. They have extremely limited dynamic range, no focus, no zoom, only 3 focal lengths and they're all "wide" compared to most cameras. They also don't have controls for exposure without messing with the GoPro app on one's mobile device, which is fun, but now you're carrying around an iPhone and GoPro at the same time.
In my eyes, for stills the iPhone blows the GoPro out of the water, no contest. For video, the GoPro does have a few nice features, it has less rolling shutter then the iPhone. It has higher bit rate recording then the iPhone as well. However, all of that is adjustable on the iPhone with app's and the new iPhone 6 Plus, has an amazing camera for both video and stills.
The nice thing about the iPhone is that it has HDR capture (high dynamic range), it has manual focus, manual zoom, adaptors to make a fish-eye look and best of all, it has a beautiful built-in screen. In my world, my iPhone lives with me 24/7 and it's been a life-changer because I can shoot anything at any time and it looks great every time!
So it really depends on what you wanna do. If you wanna shoot sports/action stuff, get a GoPro. If you wanna shoot high quality video, get a cinema camera like the Blackmagic Pocket Camera. If you wanna shoot stills, buy a still camera. I mean check out these shots from my iPhone… from my vacation this summer. Just snap shots and they blow the doors off anything shot with a GoPro.