And I still can't work out the square root of a random number as acurately and quickly as my calculator, nor the many other questions it helps you to move quickly through when you have a timed exam.
I'd advocate practicing mental arithmetic as often as possible but as whirls says not during an exam where you are permitted to use them.
If you did A-Level (or even GCSE Maths) the calculator you had will be absolutely fine - I dug mine out a few months back - 7 years old and still works fine - hate all the new ones so I dread it dying before the exams are over.