I used an HP-41 for years which finally died. I was really tempted to buy the equivalent from these folks but couldn't justify it given that I use a physical calculator once in a blue moon (and have a good HP-41 app on my iPhone).
FWIW, the best RPN caclulator app I've seen (no Android, just iPhone, sadly...) is the MathU RPN from CreativeCreek. Not only does it successfully combine the scientific, digital, and financial capabilities of the HP 10-series (12c, 15c, 16c) into one calculator, it's also one of the very few RPN apps that really gets the look and feel of HP's keyboard/UI design right. This app is literally the biggest thing still tying me to Apple's iOS, which I've finally decided I'm done with, but I really don't want to give up this app - it's one of only two I've ever paid for.
