I have only written two reviews for an iPhone app: the ND1 1.6 and the prior 1.5 release. This is a suburb app that should be on every scientific-minded/computer-literate users iPhone.
The recent update builds on the already amazing ND1 1.5. The big addition to this version is that ND1 is now a Calculator Generator, allowing one to make their own customized Calculators. How cool is that! I have always enjoyed the HP 42s, so I used ND1 to design and build a customized HP 42s+ for myself.
Even out of the box, without customization, the latest release of ND1 (1.6) is arguably the best RPN calculator app available on any mobile platform. (Trust me, I know. I have owned HP/RPN calculators since my HP-45 decorated my belt in High School. Dont tell my kids friends. . .)
The ND1 provides a significant subset of the functionality of the HP 48/50 class calculators. But rather than being just another emulator, fixated on replicating the "classics" and repeating the past, the ND1 pushes forward. It is as much of a calculator development platform as a calculator. It is extremely configurable. Not only can one reconfigure the keyboard and its keys, but one can affix ones own code to any key. Moreover, this can be done in native RPL+, in JavaScript or in combination. As a result, the ND1 is able to work with a wide variety of classes, including the traditional numeric classes, but also BigInts, Rationals, Complex, Colors and Images. (Classes, as well as other customizations, can easily be created by the user and shared with the broader ND1 community.)
Here-to-fore, my favorite RPN iPhone app has been the HP 42s (which I rate 4 *s). Though its an amazing replica of the original, the project is focused on recreating the 42s, as opposed to pushing forward. Though superbly engineered, it was done at a time when dynamic reconfigurability of the keyboard/functionality was limited. In contrast, the ND1 is easily reconfigured. Currently, I am taking a Chemistry course, so I wanted a 42s type calculator that has physical constants built-in. I was easily able to reconfigure the ND1 to do so. Moreover, for my day job, I wanted to include a core-set of financial functions, which I was also able to easily add.
As impressed as I am with ND1 in its current incarnation, I believe the best is yet to come. I believe that the developers of ND1 will continue to focus on configurability and expandability.
Lest you feel intimidated by all of the power of the ND1, dont be. As I mentioned, it is ready to go right out of the box, but its nice to know if one wants to make "tweaks" that these can be easily accomplished.