I was playing with some lenses and my iPhone 6 and found that some were good for Macro photography. But I needed a way to hold the lens firmly in place. John Umekubo had a good friction fit design posted on 123d's models site that I adapted to fit my iPhone6 Plus. It fits over the existing case, but is flexible enough that I believe it would work without a case too.

As you can see in the photos, you can get really close to an object, but as with most macro photography, the depth of field is shallow. The lens I used will show a vignette on the iPhone preview, I zoom the screen a bit to make the view expand to the entire screen.

I adapted the lens holder to fit a lens that I scavenged from some discarded CCTV cameras. It might be worth a call to some local security companies in your area to see if they have any old analog cameras and lenses they could part with. They are practically useless now, because everything is going digital. You'll have to change the adapter to fit whatever lens you come up with, I'm sure. So I've included the 123dDesign file.

Remixed from 123D design by John Umekubo

Original design is here: http://www.123dapp.com/123D_Design/Macro-Lens-Holder-iPhone-6/4114876
I had to make his adapter wider, and put a 10mm hole further down the back of it. This allowed the lens I had to friction fit the hole and center itself on the existing iPhone camera lens.