The combination of low light and long time exposures creates a limit on how many images you can capture. A singe final image can take up to 20 minutes to capture especially if multiple exposures are needed. This means more time thinking about and composing images.
For these long exposures the camera sensor has a tendency to highlight flaws in its image quality so it is always important use a low ISO and to ensure the in-camera noise cancellation feature enabled. This seems counter intuitive using a low ISO as it makes the exposure all the longer however it has the advantage that image flaws created are mostly systematic in the sensor which allows the noise cancellation process to work better. The disadvantage is that an already long exposure, for example 5 minutes, then requires the same amount of time again for in-camera noise cancellation. At least you can spend the next 5 minutes composing your next capture.
This image is of the Adelaide Convention Centre and the Hotel Intercontinental with a waxing moon overhead.
Photo: Robert Rath, '366 Days of 2012, Day 269 - Urban Sentinels'. 3.2sec(+-2stops) f/5.6 ISO160 15mm hdr