Most of us won't have eaten true wasabi. A relative of our more homespun horseradish, the Japanese root grows next to highland mountain streams, takes two years to reach maturity and is very perishable and therefore expensive to ship far from its growing patch. (The wasabi served in supermarket sushi is probably good old horseradish and some colouring.)
Powdered real wasabi will still be expensive, but should have the benefit of being used when you need it. True wasabi will contain wasabia japonica, while faux wasabi (and most of us will have to make do, won't we?) will be called Armoracia rusticana.
The roots will keep in the refrigerator for a few weeks, and can be grated as needed. Once exposed to the air, the flavour deteriorates quite quickly.