Here are sample images taken with the Pentax 110 50mm F2.8 lens mounted on the iPhone. First two shots were taken in normal photo mode with iPhone 6 plus – there are black corners similar to fish-eye lenses, but this is much better than any other lens tested.
When shot in square mode, the black corners disappear. Or the centre of the image can be cropped into (approx. 2980×1920) to get rid of the black corners.
Added images 10/2/15:
These images of the moss growing on the wall definitely looks much better than those I took with the magnetic macro lens.
I would say that this lens is lighter than the Moment telephoto lens but about the same size. It gives me a better quality macro lens than the small magnetic clip-on one I have, with much better quality images. And as it is easily available on eBay or through second-hand camera dealers, it should be fairly easy to get one to try.
It does perform very well, and gives more range (35mm focusing range) and focuses further from the subject than the macro lenses currently on the market. I have read that the best macro lens currently available are from Olloclip, but these are not cheap and are not flat-field either. So my DIY has saved me some money and given me a very good macro lens. And I am not sure if they can actually come up with a better macro lens than this.