Never.
I've never bought a roll of slide film in my life, except for the time someone sent me out mith their SLR to take slides of some artwork for them. (Oh, wait; bought a roll of Fuji 120 slide film, once, for a photo club nighttime project.) Hell, I barely use color.
It's funny, though, because my old man was a monster slide photographer from the 40s through the mid-70s, and I saw gobs of nice, sharp, correctly-exposed Kodachromes and Ektachromes as a kid, all taken through a 50mm f/3.5 Color-Skopar on a Prominent. He gave me that camera around 1981, and I promptly banged a roll of Tri-X into it and went crazy.
I guess I've always preferred the immediacy and versatility of the pocketable, instantly-displayable snapshot.