I ruin more Velvia 50 than any other film I shoot, but when it works, it produces some of my favorite shots.
Since it's meant to be projected, the image – particularly on Velvia 50 – has a lot more pop to it. Colors are more saturated, contrast is amped. Negative film stock like Portra or Ektar are meant to be scanned and corrected on the computer and contain a lot more latitude. If you get Velvia wrong, there's much detail to recover. The exposure latitude is razor-thin.
Because it's a slow film too, it requires a lot of light which sucks because it's a great film for the skies at sunset or dusk. I finally got a cable release this week for my Rolleiflex 6000 system. Before I had that, I lost a lot of pre-dawn Velvia shots to motion blur because I just couldn't punch the shutter with enough finesse to not cause camera shake.
I have tons of Velvia 50 in 35MM rolls at the moment, but haven't used it yet. It's felt like none of my lenses could live up to the requirements, but I just picked up a Leica M3 and a same-era 50mm lens for it, too. I'm waiting for a meter to come with it, but I'm planning on seeing how the smaller format Velvia works soon.