As title, do you use any scientific calculator apps you're happy with? The stock Android one is OK for basic functions, but that's about it. Anything other than e.g. basic trigonometric functions is either cumbersome or seemingly impossible.
I've just tried a couple of highly-rated ones and nothing would have me leaving behind my calculator, from only having a single display line to using some funky input methods for functions. E.g. IMO, 2cos(20) makes more sense entered as "2 cos 20 =" not "2 x 20 cos =", and since there's no second display line, it's hard to have confidence in the output as there's no way of checking what it's actually calculated.
Another interesting one I tried is the MyScript calculator, which allows you to handwrite calculations, and the recognition seems pretty good from my playing with it. It even auto-balances calculations so you can write 2x?=8, and it will give you the answer 4. However writing more than a few digits on a phone screen with a finger can get awkward. I can see it being better suited to stylus and/or tablet use though.
TBH I can't see myself replacing a proper calculator, but it would be nice to have a decent app on my phone for when I'm without one.