In my own experience, I have found that K64 is not a particularly easy or forgiving film to use, and I don't use it with flash as it does, with my equipment and working, give harsh and blue-ish results. No doubt this could be tamed with some experimentation with exposure, filters, etc., but it's not worth the effort for me at least.
I think that the feature of Kodachrome generally (particularly K25) is the delicacy andaccuracy of the color rendering, which is not necessarily the most pleasing result in every case. Straight pictures in noon sunlight and many landscapes can look dull, but delicate shades in warmer lighting, and bright colors will be rendered remarkably well.
If I just want to shoot a few slides for record purposes, without worrying too much about lighting conditions, I'd use Astia, which is much more forgiving (and is a great film in itself, my 2nd choice if K64 vanished). But if I have lots of time, and conditions are right, I'll usually use Kodachrome with the confidence, if everything is spot-on, I'll get that unique look.