I like the 70s calculators. They are so simple to work and generally do not have the complexity overhead of modern calculators.
My first calculator was a Sinclair Scientific, which I build from a kit. I think it cost me £19.99. Construction was easy but it lived up to the Sinclair mantra of 'cheap and nasty'. It worked, was not very accurate but easily fitted in a shirt pocket and looked cool with purple LED digits. It had so much use that it just wore out and dropped to bits.
I lusted after the SR-52 but just could not afford it.
I was lucky enough to get the smaller SR-56 in 1977 and this has been my mainstay calculator up until recently. The SR-56 was programmable and an amazingly powerful piece of kit in the days before affordable personal computers. Programming the SR-56 made the transition to 6502 code easy.
Now my favourite calculator is the SR-51A. It is not programmable (I would use BBC Basic or use a spreadsheet for complicated calculations that needed programming.) but has all the functions I need easily at hand. For a quick day-to-day calculation, though, I use the TI-66, a LCD calculator which is similar in some ways to the TI-58C
A few weeks ago I bought my first graphing calculator (TI-85) , which is an amazing piece of kit. However, using one of these for mundane scientific calculations is really difficult compared to the calculators of the 70s. One has to work out HOW to do the calculation before trying to input it into the calculator. For example, enter 29 and then find the sine, cosine of the result followed by the tangent of that result and then go back with Atan, Acos and Asin, is really horrible on the graphing calculator - it has to be done in one operation - Asin(Acos(Atan(tan(cos(sin(29)))))). With a 70s calculator the operation is 29 Sin Cos Tan Atan Acos Asin; the intermediate results appear in the display between pressing buttons - much simpler to my feeble mind.
Mind you, a calculator I'd really like is the HP-16, the programmer's calculator but they are so darn expensive.