I have had a Casio fx-451M calculator since I went to secondary school. I bought it in 1987, when I was 11, and I've never found another calculator that I like better. Unfortunately, it has something of a design flaw in that there is only the thinnest of connectors between the two halves. And the first thing most people do if they borrow it is to fold the "case" back through 180 degrees - which makes me scream at them.
My calculator would still be working absolutely fine if not for other people, but as it is, it's been abused over the years. The connector between the two halves has failed so some of the functions only work some of the time. This would be fine, if they didn't include Memory In and Memory Recall. Also, the case is pulled off the edge in several places, and the sticky substance inside is all full of poppy seeds (an occupational hazard of anything I own). So I decided I'd better start hunting on eBay to prepare myself for the time when my calculator finally stops working altogether. And I found one!
What I like about the fx451-M is the physical constants. It has Avogadro's constant, the speed of light in vacuum, the actual mass of a proton and electron (in kg), the charge on a proton or electron (in C), the molar gas constant, Planck's constant, the permittivity of free space, g, and some other constants that I never used. Apparently some modern Hewlett Packard calculators also feature these constants, but through a menu rather than a single keypress. I use NA, R and h enough that I prefer to have them permanently visible - I use each of those three far more often than pi!
So I'm happy, and now I'm wondering if it would be greedy for me to continue scouring the ads for more fx-451Ms. Because, y'know, I can reasonably expect to live for at least another 40 years, and if this one's lasted 20 years... I'm not going to allow anyone else to use the new one, when it comes. I'll get a crappy modern calculator for the students to use when they forget to bring theirs.
Also, dear lord, this is "vintage" - it's the calculator I had for A-level. Selling for US $3! Mine cost me over £100, plus another £40 when I dropped it and the LCD smashed. For some reason, this makes me feel old far more than the fact my Game Boy Advance SP has a more powerful version of the same ARM processor that my Archimedes had.