The HP-15C is probably one of the best product ever made as well as the HP-12C and HP-41CX/CV. In every aspect in mathematical analysis these instruments attain flawless results while any one of these calculators/computers I consider ergonomically perfect. Just hold any of these and test and you immediately realize the enormous amount of thought that went into the design and function of the product while I do not need to mention the RPN logic, which once learned, you will never go back to whatever you were previously using. True, these instruments are some-what antiquated, many will find themselves going back to using these near-perfect tools.