When should we use Radix sort?

Question

It seems Radix sort has a very good average case performance, i.e. O(kN): http://en.wikipedia.org/wiki/Radix_sort

Yet it seems like most people are still using

Answer 1

Radix sort is harder to generalize than most other sorting algorithms. It requires fixed size keys, and some standard way of breaking the keys into pieces. Thus it never finds its way into libraries.