Shooting in low light can be a challenge. One has to strike a balance between using a high ISO, or a slow shutter speed to capture an image. The choice you make can make all the difference in getting a memorable image. Using film, a high ISO meant 400 most of the time, and in the late 1980′s, the limit crept up to 3200 (if you liked large amounts of grain in your image). I tended to keep my film selection to the finer grained ISO’s. I rarely shot at 400 ISO, preferring to shoot with 25 to 100 for most of my early images. A heavy tripod was an essential part of my equipment. Now that I am shooting digitally, I tend to keep my ISO around 400 and I like to hand hold most of my shots. My gear is lighter, and my knees appreciate the lack of excess weight.
The biggest advantage of long exposures is better color saturation. Slot canyon photographs are prime examples of what long exposures do best. Notice the colors and richness that a long exposure provides. This image was shot using 100 ISO film, f-16, and about 37 seconds for the exposure. 
I would probably use similar techniques had I shot this using digital equipment. The most important thing to keep in mind when making long exposures with a digital camera is that longer exposures introduce more noise in the image. This is because the longer the sensor is energized for capturing an image, the more it heats up. The extra heat is seen as Gaussian distributed noise in your final image.
Images made using higher ISO’s in digital cameras also have a larger amount of noise. It is caused by a sensor that is being driven at a higher potential (voltage) than the lower ISO images. Thus you will see more noise in the lighter areas with a high ISO than in the darker areas. This non random noise is similar to that of film grain and can be exploited in the creation of old style images and Sepia tones. This image of some swing dancers is indicative of a low light level, short exposure, high ISO digital shot. 
Your selection of ISO and exposure should always be made so as to achieve your desired image. Get creative and play around with them. Nikon, Canon, and every other camera maker out there do not have your vision in mind when they set their camera’s Auto settings.