I did this last year, bought a very nice bow mount for only about $40 I think that part works great.
The trouble I had was starting and stopping the video - My iPhone has a passcode lock on it (IMHO like all of them should). In order to start the video you have to unlock the phone. That presented a real problem because it was an extra task to do when an animal was coming in.
I did manage to video a couple (a hog and a small buck), but I don't think I'm going to be repeating that this year.
The GoPro is so much better suited for this type of video.