I'm not sure how many licenses OpalCalc is going to sell at $15, though, even with evangelists such as yourself.
I rarely evangelise -- I am told often enough that my taste in software is "a bit weird" that I certainly wouldn't want to insist that a particular thing is best. But the traditional calculator, however good they are (and my experiences of the things goes back to the 1970s and I still own several) is specifically designed to be finger-friendly in a relatively restricted space -- something that computerised calculator programs don't, it seems to me, need to emulate.
I tried a couple of the wabbitemu TI emulations and was thoroughly impressed with the quality of them -- and I completely get that a smartphone host for one of them would be a good idea. The restricted space and keyboard facilities apply again, so it makes sense.
For me, Opalcalc sits nicely between the input complexity of the calculator (infix or postfix? Do I start with a number and apply a trig function to it, or start with the function as if I were writing the formula on paper? How do I get at the stats functions again?) and the big iron of the fullblown spreadsheet, and it needs me to have a full-size keyboard I can use with it without having to grope for symbols.
And some of the things it can do would need Google, otherwise. How many days to Christmas? (@ 25 dec 2015 - @ today as whole days) What's $15 in euro? ($15 as euro) How many centimetres in 12 fathoms? (12 fathoms as cm)
And the developer is still tinkering with it, and that $15 is a lifetime license. Personally, I think it's worth every cent. But then, I'm a bit weird.