I used to use the app iSymDVR2 for my Iphone 5S, works really well and has lots of settings to play with :) (not sure if it's been updated for the 6 yet though)
Now I just use a cheapo dashcam which seems to do what I require of it....
And looking at some of these videos on here it makes you realize just how bad alot of drivers really are out there, so be safe everyone