With the advent of digital photography, many photographers are venturing into long shutter speed photography. They are finding that you don't have to wait for night fall or a solar eclipse to get a long shutter speed; you just need neutral density filters. Neutral density filters range anywhere from one stop to fifteen stops. Thus, you can reduce the light entering the camera anywhere from one stop up to fifteen stops of light.
This means that if you are shooting in the middle of the day with full sunlight you would have to potential of getting a 4 minute exposure with a 15 stop filter if your starting point is ISO 100 at f/16 and 1/125 shutter speed (this is the Sunny 16 Rule). The exact calculation would be 256 seconds = 4 minutes 16 seconds
If you have ever shopped for a 10 or 15 stop filter, you would know that they are very expensive. As a result some will recommend that you just buy a 5 stop and a 10 stop filter and stack them. This would give you more flexibility and it would be less expensive; however, there are issues associated with stacking circular filters. If you are shooting very wide, you will notice that the stacked filters cause a vignette on the edges of frame. See image below. Another problem with stacking filters is there are more surfaces to keep clean so if you have dirty filters you will compound the problem. The last problem is simply image quality. The best image quality is gained from using the lens without any other filters on it. All filters downgrade lens glass to "some" extent. Most of the time this "downgrade" is purely an academic/scientific discussion as it is difficult to detect even with a very large enlargement of the image, but the fact remains that it does indeed do this. Therefore, if you are stacking filters you are compounding the problem. If you have a low quality filter and you stack your filters it will definitely be visible.
So although the financial savings and the flexibility is appealing, it may not be worthwhile unless it is unavoidable.