I have used HP calculators for may years. My current ones are the now superceded 15-C, 11-C, and 11-C calculators which I've had for around 20 years and use on a daily basis (the original batteries have required replacing in one of them). I buy reverse polish (there is no "=" sign) calculators but I believe other types are available. My realtor and financial manager both use the HP-12C business calculator. Rarely seen in office stores (such as Staples) they can be bought direct from HP. [You can also download reverse polish calculator app from the Apple store for use with an iPad.]
Many people in the scientific/engineering community (including me) are long-time fans of HP's RPN calculator line. I've had an 11C since the 1980s, and always regretted not buying a 15C, which was discontinued in 1989. Fortunately, the 15C had such a devoted fan base that HP brought it back for a "limited edition" run in 2011, and I got one then. I also have a 32SII and the monster 48GX and 50G graphing models. I paid around $100 to get two memory expansion cards for the 48GX: one with 128 KB and another with 512 KB of extra RAM. It was a different world in the 1970s and 1980s -- at that time, nobody had a computer on their desktop. So you needed the most powerful number-crunching calculator available, and those were the HP RPN models. But nobody relies on calculators for complex calculations any more -- now you just grab your mouse and double-click on the Excel icon. There is little demand for high-end calculators, except among high school students who are learning math, and TI has locked up that market with non-RPN graphing calculators. Asking people about their preferences in calculators today is a bit like asking them whether they prefer Pickett or K+E slide rules.
Or other students, and some of us still are that. For one of my present adventures, pre-health General Chemistry, we're barred from using programmable or graphing calculators on exams. I invested some time in shopping for the best calculator I could find within these constraints, and found the TI-36X Pro. Now I evangelize it to my classmates to the point that I'm almost a TI campus rep. For future applications where programmable and graphing functionality is okay, and might be helpful – there are no such constraints, for instance, for exams for the physics courses here – I've started to investigate the TI-Nspire series. Thoughts from the floor?
Technically, "programmable" and "graphing" are different things. Some exams, like the NCEES professional engineering and surveying exams, allow programmable calculators, but not graphing calculators. In this situation, the most powerful model is the HP-35S, which can be used in either RPN mode (the "power user" option found on classic HPs) or algebraic mode (the clumsier, but more familiar, option used by TI and most other brands). If you are interested in graphing calculators (all of which are also programmable) then I would look at the HP Prime, a brand-new model that is claimed to be the most advanced graphing calculator on the market -- including a touch screen. Have not used it (or the Nspire). Graphing calculators are nice for symbolic math (calculus, differential equations, etc), and they sure look cool. But if you just want to plug numbers into equations and formulas to get answers, you may be better off with a non-graphing programmable like the 35S. It may be easier to program, and it is certainly cheaper.
If I'm at a UNIX/Linux command line I will use the 'bc' utility/language. If not, I will use the calculator on my Macbook.
The default OS X calculator can be set to RPN mode, and it isn't bad. Or you can run 'bc' or 'dc' in Terminal. However, I personally like PCalc on the Mac.
Here and there, I've been learning to use an abacus. Not a joke. I don't actually own an abacus; there are several apps available that use touchscreen capability to emulate them. I might eventually buy one.
