I love my TI89 Titanium, but my 1st scientific was an HP15C. I love the reverse polish notation of the HP's. I found this HP48GX at a flea market in my hometown this week for 25$. Price on Amazon? $150.00 !!!!!
I don't remember exactly which calculator I first had but it was very much like this and my "big" Christmas present in mid 1970s this is the HP-35 Calculator from 1972. Look at the cost! It is still my favourite calculator of all time. Great keyboard, and used Reverse Polish notation logic for entry, which seemed so logical.
HP 12C Financial Calculator HP 12C Financial Calculator The HP 12C Financial Calculator features built-in financial functions and statistics, uses Reverse Polish Notation (RPN), more than 120 built-in functions, including register-based cash-flow analysis, 10-character, 1-line LCD display, Device measures 5.0 x 0.6 x 3.1 inches (WxHxD).It has 10-character, 1-line LCD display. http://www.newofficestore.com/hp-12c-financial-calculator-3/