I've shot a lot with Velvia, love it, great for landscapes, super sharp, colors pop out at you.
Kodachrome was best for portraits and sunsets ... wonderful warm tones.
I've read for a long time that for archival purposes, Fuji was suspect ... that the slides weren't thought to last as long as the older Kodak film. It hasn't been around long enough to know for sure yet, but from what I've seen so far, no problems.