I tried this out quite a few years ago and didn't care for it (although I can't remember why). I tried it out recently and didn't care for the appearance, but I found that it was the only RPN calculator that works the way I like. I was looking for a replacement for J.M. Baker's 'RPN Calculator' which isn't developed anymore. I played around and found that the appearance, size, etc. are highly customizable, so now it looks great. PCalc has a combination of features that I couldn't find anywhere else--RPN, stack view, keyboard shortcuts that I like... I initially thought $19 was high for a calculator, but I soon found I couldn't live without it and registered. I took advantage of the offer to get the iPhone/iPod version too, for only $.99 more than the price of the desktop version. I keep finding features (like tons of physics unit conversions, for example) that are saving me a lot of time. It appears to have outstanding support as well. Best app I've (re)discovered in a long time.