I bought mine in 1987 and it got me through university, and still runs on the same set of batteries it came with 23 years ago. The HP-12C financial version of this calculator did so well that it continues to be sold to this day.
If you're unfamiliar with this calculator, you can find a scanned copy of the Owner's Handbook (and the Advanced Functions Handbook) online. There is even a Petition to Bring Back the 15C that you can sign.
If you've ever used this calculator, or one like it, I hope you find this simulator entertaining. Feedback, bug reports, and even contributions are welcome!