How are the keys on the HP Prime? As good as the ones on the HP-67 and HP-41?
They're OK IME, although on the original version of the Prime, the contrast on the colouring of the keys made it almost impossible to use without good lighting.
As a calculator, it's interesting, but frankly if you're going to do anything outside of the classroom or exam room, you'd do it on a proper computer: the limitations of the non-qwerty keyboard interface and small screen are its biggest drawback for real, practical engineering purposes. The one exception to this is that I found it of some limited use on commuter train journeys where you often can't practically get a laptop out: I've done a fair bit of complex DSP modelling on the Prime at various times.
Like many non-trivial calculators, you really need to use it reasonably frequently to keep current and use it to its greatest capabilities.
I think that's why I'm still quite a fan of the specific Casio line FX-100D/115D/570xD/991D: they're just so very straightforward to use, offering immediate execution, and they persist the engineering display mode.
Although I have an FX-61F, I never found that I had much practical use for it, preferring the FX-100D etc mentioned above.
In recent years, I've found myself drifting over to RPN, and have become a bit of a fan boy of the HP Voyager series, although I find there displays are rather limiting compared to modern alternatives. The form factor is just my cup of tea though.
In practice, my daily drivers nowadays are either the WP-34S or the DM42: one I have with me at work, and the other at home.