My mom made this ALLLLLL summer long, and I remember as a kid knowing that summer was ON, the minute this pie came out of the freezer.
She didn't always use key limes, but regular limes worked just as well. Regular limes tend not to be as sweet which is great if you want a more tart pie. But if you want it sweeter, add more sugar, about 1 tablespoon at a time.
Taste the filling before you pour it in the crust to be sure it tastes the way YOU like.