> While I own a few HP calculators, my favorite is still the HP-28. Not sure why it isn't as remembered as the other models,

I've been a big 48 fan for years. (The 48 and the 28 are very similar.)

I think they both suffer a bit from design overreach. The devices are very capable, but the capability can make them more cumbersome to use than earlier, simpler devices. And then the 28 has the problem that with the flip out case it isn't really a handheld.

The result is that compared to contemporary calculators they were more difficult to use and compared to contemporary computers they weren't nearly as capable. So they fell into a gap, and there wasn't enough life left in the calculator market left to fund a way out.

Another way of looking at this is that the 28 was a v1 product, the 48 was a v2 product, and there wasn't ever the v3 product (where v3 is usually where products hit their stride). I don't really count the 49 or 50 because by that time, the calculator division had been radically restructured and those products were the output of a very different kind of engineering process. (No offense to Jean-Yves Avenard, who did great work within the constraints of the time.)

The flip case is a large reason why I prefer the 28 though. When I just need a calculator, fold it back and it's a handheld calculator. When I want to program, fold it out like a book and have a more extensive keyboard. Best of both worlds.

I should find a 48, don't have one of those. I have a 50 but don't like it much. I keep it in the office for basic use.

> The flip case is a large reason why I prefer the 28 though

I can see the split case might have some benefits. It wouldn't work the way you describe, but I wish they'd put scientific calculator hardware in the 200LX case. Nice display and great keyboard. (The 200LX _did_ have a built in program that acted a little like an 18C business calculator.)

> I should find a 48, don't have one of those

I have both a 48SX and a GX. I like the form factor of both better than the 50, although the hardware on the 50 is much better. (If I remember properly, they replaced the custom Saturn CPU in the 50 with an ARM chip running an emulator... it's overall much faster.) Neither of mine get much use, but they do have lots of sentimental value.