I've got it on my iPhone. Wouldn't rely on it to the same degree as a proper GPS on the bars to be honest, and I'd still carry maps anyway. The tracks it draws are rubbish too. It doesn't take way points often enough so you end up with a rough approximation of the ride recorded. It warns about this on the app description to be fair.
My geko 201 cost £60, and it's been dropped in a river, battered in falls, covered in mud, grit, soaked in rain, and frozen. Amongst other horrid things. Not sure an iPhone would be up to any of that abuse, and it's a more expensive thing to break too.