I use B.iCycle as a replacement for my old school bike computer and I'm very happy with it. I have a bike mount on the handle bar
With the type of mtb biking I do, there is no way I'd mount my iPhone in a clamp on the handle bars. I have a Cateye there and wouldn't dream of subjecting the iPhone to those sort of shocks 'n vibes. I'll use a conventional map if on a trail I don't know and the iPhone strictly for record keeping, safely tucked up in my Camelbak.