If I'm shooting color, I'm almost always shooting Velvia (I do shoot a little Ektar every once in a while). I shoot exclusively 35mm.
Why 35mm? I take most of my pictures in places where a larger camera system can't go -- up mountains, down rivers, across the middle of the desert, all on my two feet or in a small boat (usually a canoe/kayak). So either I don't want to be lugging a heavy bag around, or it physically wouldn't fit in the boat.
Why Velvia? This is more difficult to explain. I wish I could print it. All the time. But I've tried out just about every C-41 film that there is, and none of them have given me the results that I want. I really only use color for landscapes, and even on a computer screen or as a hybrid print there's no other way to get the colors I want. I like the way that Ektar handles certain situations, especially ones that have a lot of green in them, like a New England forest, or the odd occasion that I need to have a color picture of a person. But 90% of the time I go for Velvia. If I'm shooting color I'm shooting it for the color. Velvia gives me the colors I want.
And maybe I'll be able to make real prints from it in the future. I'll have the positives. That's another advantage of Velvia: Fuji says it will archive for hundreds of years under the conditions I store them.