Your default ISO should be around 400 for most shots. This will allow your camera to refresh quickly.
The lower the ISO the slower the film. If you can image it, the slower the film moves the more detail it can absorb.
On occasions you will find it beneficial to set your ISO higher. When you are taking photos in poorly lit areas or though thick glass.
The photo featured here was taken with a higher ISO, please be aware you need a VERY steady hand, a mono-pod or a tri-pod. Because the film is moving slowly, you cant let the camera move.
Play around and see what works for you!
Cheers,
Lin