[Trashcan]

Yesterday I picked up a Nikon D40 as an introduction to digital photography. Before that, I was using my Nikon N75 35mm. With 35mm, I noticed that a lot of professionals recommend Velvia (50 speed.) I always shot with 200-400, but I never noticed a difference due to my camera automatically adjusting exposure based on film speed.
So now that I have the flexibility to change my speed (ISO sensitivity) on the fly, what are the practical uses and effects of different speeds of film? I know what it _is_, but I'm looking for practical reasons for changing.

Anyone care to share some experience?