Customer Rating: 
Summary: Powerful, But Not Easy to Use
Comment: I have had this for almost 3 decades (there is a more recent version of it, but I have not seen it yet). Recently, I determined to learn every capability of this calculator (I only use the more obvious functions). There is an awful lot of subtlety, and it is impossible to use the advanced functions without the Owner's Manual & Problem-Solving Guide. I was about to take it down a star or two for its complexity, but this was really not any worse than the other financial calculators I have used, so par for this. Just make sure that you keep the manual stored with the calculator: you're gonna need it. However, I decided to give it top rank; at least it possesses all those capabilities (not that you will ever figure out to make it work). Despite its age, I still like and use it.
A small sampling of obscure things:
* "12x" key not only times by 12, but also puts the result into "n"
* "12÷" key not only divides by 12, but also puts the result into "i"
* "f" and "." puts display into scientific notation
* does not have a "log" key, but it does do common logs (you just have to know the magic word)
* does not have a 10^x key, but has a "Y^X" key
* holding down "-" and pressing "on" clears all registers (including the programming ones) at the same time
* the date input format and payment convention (beginning or end of a period) are not reset when you turn it off
* "prefix" has 2 different functions
* sometimes you have to press the financial keys twice
* difference between "i" and "INT" keys
Heaven knows what all else.
FINANCIAL FUNCTIONS
Some of these are obvious: statistics (sigma+, std dev, and mean), or financial (n, i, PV, PMT, FV; just remember the cash flow convention); these you can pretty much figure out just by looking at the keys. Others, however, you will stare at thinking "duh?": 2-coordinate linear estimation, bond, depreciation, discounted cash flow, weighted mean, sample vs. actual mean for the statistics function, amortization. For these, you will have to study the examples in the manual. There is a cheat sheet on the label on the back, but these are rather cryptic and not always helpful.
PROGRAMMING
I skipped this section. Yes, you can write your own mathematical program at the register level. But a spreadsheet is much easier and faster (remember: this thing was made before there was a such thing as a "personal" computer).
SOLUTIONS
Because of its programming capability, this calculator has many more capabilities than just the hard-wired ones. The manual and the optional solutions manual have literally dozens of different financial calculations that the 12C can do. Problem: you have to manually key them in, and some of the programs are quite long. Also, it can only hold one program at a time (course, since the memory is continuous, the last program you keyed in will stay in memory even when you turn it off). If you want to do a different program, you will have to wipe out the old one and key in the new one.
Customer Rating: 
Summary: "Nothing has changed since then" -- Not quite
Comment: I have one of these. But I'm not really a finance person---I'm a scientist/programmer/engineer. So I also have an HP-15C, the legendary scientific calculator in this same series. I have also owned HP-35, HP-28C, and HP-28S calculators. HP always made wonderful calculators, and the 12C is no exception. RPN is really the way to go for a pocket calculator, and the form factor is great too.
So why only three stars? Just because of the "nothing has changed since then" nonsense. My old HP-15C is stamped "USA"; I have a newer HP somewhere stamped "Mexico". And now "Made in China". Not surprising and nothing to get upset about, if it were in fact the same product.
But it's not. Holding my HP-15 in one hand... I bought it new in 1986... shake it... it's like holding a small, thin brick. My 12C, which is new.. do the same... Rattle, rattle, rattle. That's right, the keys are loose! On a brand new calculator. Overall it's not even close to in the same league as its old cousin. I don't think it's the Chinese people's fault but some cost-cutting mandated by HP headquarters, but the end result is the same. Crap.
If you need a financial calculator, go search eBay first. Find yourself a used HP-12C, made in "USA" or "Mexico". Only if that fails, get one of these. The functionality is the same, but do you really want something that screams "plastic"?
