I think ND1 on iPhone (iOS 7 5s) is great! I have used HP calculators all the way back to the original HP 45. On the iPhone, I have tried most of the HP/RPN App's. Until now, my favorite had been the RPN 42s by Byron Fooster. ND1 is so much more capable, that I would really want it to become my only calculator. Though, the functionality of the ND1 is clearly superior, the UI and user experience is much better on the RPN 42s. The 42s is absolutely gorgeous. The colors, the 3-D richness of keys and the size of the keys are such that it's hard to remember I am not using a "real" HP calculator. Part of the attraction of the early HP lines were that they were masterpieces of engineering. Well before Apple existed, HP was designing products that you just needed to own. (Ok, not everyone . .)
I've read the forums and tutorials and have tweaked the skins, but nothing I have found comes close to the 42s in terms of design. I would challenge you to focus some more attention on the UI. For me, the most important feature would be larger keys. I would not want you to significantly reduce the stack size, but at a minimum, consider removing the Date/Time Header from the screen top and take advantage of the unused 3/8" on the screen bottom. I would also like to have different colors for the Alternate Key labels. Also, is it possible to increase the volume of key-clicks or vibrate the phone slightly with each key click ? (Not totally sure on how the vibrate would work.) The more cues you can give a user that the keys were hit the better. The original HP 'a all had very satisfying key-clicks. In short, take a hard look at the 42s as inspiration.
Please take this in the spirit in which is provided. You have a great product. I 'm just sharing my thoughts on how the ND1 could become a fantastic product for me.
Thanks and keep up the great work.