95-99% of the time, I'd actually prefer the built in android calculator, because the buttons are bigger and I'm only doing simple arithmetic. I'm perfectly willing to sacrifice the remaining times in exchange for not having to carry another thing.
I am probably two orders of magnitude faster[1] on a real TI than on an emulator. The muscle memory there is deep, and doesn't translate to my phone at all.
The user experience is many times better on a real TI then in an Android emulator or even in a computer. For some quick math calculations a computer or Android calculator is find. If I'm doing a lot of math problems I rather use a TI. The usability is just way better.