I own a handful of TI calculators (TI-{80, 83, 83+, 86, 89T, NSpire CAS}, possibly a few others) but actually use a TI-89 Titanium emulator on my Android phone. I no longer do anything remotely interesting with it, but I prefer it to the stock calculator since I know how it does order of operations, it shows fractions, it's easy to get old return values, etc. Don't tell me if the stock calculator now does all that; the TI-89 ergonomics are burned into my brain. (I was in an advanced math class which got me using a TI-80 at the age of 9)