While GoPro gets all the attention, I've found a better way to record action-packed video. Instead of paying $300 or more for a dedicated camera and accessories, why not use your existing iPhone? That way you can edit your video right on the device that recorded it, using iMovie and other video FX apps. And of course you can share your video using iMessage, Mail, AirDrop, and more.
But, you want to keep your iPhone safe during your video exploits? I've used the Optrix case for my older iPhone 4. It's waterproof and can even protect your phone from getting crushed. If you have an old iPhone 4 or even an iPod touch, you can use it with the $100 Optrix case, with helmet attachments, and spend a lot less than you would on a GoPro. Other mounts are available at a nominal charge.
Optrix recently introduced cases that support the iPhone 5c as well as iPhone 5s and the iPhone 5. What is really interesting are the swappable lenses. Depending on your activity, you can use telephoto, fisheye, or macro lenses. This is something you might pay $60 or more for, and it's included with the Optrix PhotoProX case.
I've used Optrix case with my iPhone to record video from my motorcycle. I love that I can use the big screen to preview the video, instead of a tiny screen on a GoPro. Plus Optrix offers apps that let you record telemetry data like your route and speed, and embed that within your video! Check out this video I made using the Optrix case and my iPhone! It rained that day, but I didn't have to worry, thanks to the waterproof Optrix case!