Hi;
I also have three working HP15C, one of them bought as new (I'm the first and 'last' owner, hope so), the other one bought one year ago from the first buyer, too (at least it came with the orignal box!), and a genuine 'frankencalc', an HP15C built with an orignal HP11C box, main(key)board and nut processor, while the extra RAM/ROM chip and the R2D2 (RAM/ROM Display Driver) came from an HP15C. I carefully removed the HP15C's keyboard faceplate and placed it in the HP11C box. The reason was that the HP11C box was a newer, one-piece main/keyboard model with a deffective LCD while the donator HP15C was not working because the flex circuit assembly had so too many broken copper trails. Well, I ennded up having a genuine, fully working frankencalc.
About the usage: the HP15C had all I could ask for in a single calculator when I was at the University. As an Electrical Engineer, dealing with complex arithmetic and matrices was a must, numerical solve and inetgration being also too handy when sudying functions. I keep one of them as 'cold stand by' (stored with no bateries inside; I just 'flash it in' from time to time to see if all is fine), the frankencalc as the 'daily use' and the third one to test some specific procedures so I do not need to erase its memory or to start it all over later if I need something else.
In this sort of survey, I'm always between the HP42S and the HP15C, but I'd certainly consider what was achieved at the time the HP15C was introduced as being more significant than the same achievements with the HP42S. The HP42S is, indeed, a lot more powerfull calculator, but the HP15C is fancier... And I am not sure if there was any other calculator in the market at that time that was able of dealing with complex numbers and matrices with its own integrated features. That was something new.
Cheers.
Luiz (Brazil)
Edited: 1 May 2006, 2:58 a.m. after one or more responses were posted