Hi @timtam, I split the topic as graduated ND filters are very much a different subject than moon photography.
I love graduated ND filters. Yes, I use them when appropriate for the situation. It certainly helps to keep the foreground in high-contrast situations from being overly underexposed.
I prefer filters over lightening Shadows in LR or photo blending in PS. However, I’ve found the circular filter that I currently own does not work in very many situations. Very rarely is the horizon through the center of my compositions. Nor is it common that my horizon is basically “flat”; it seems as though there’s always a mountain or tree or rock or other element protruding up beyond horizontal center of the frame and in such cases the darkening effect on such an element is very noticeable.
It is possible to fix this in LR with a brush, but it’s hard to keep it from looking processed. Therefore, I begun to not use the filter as much recently relying more on the LR processing. However, the graduation on my filter is not hard, so it is forgiving and I have used it often in situations where the horizon was not in the horizontal center of the frame.
While the circular filters are extremely convenient, I would recommend you go with a rectangular filter system with a holder that attaches to front of the lens.
This is the filter that I currently use: http://www.amazon.com/Tiffen-Graduated-Neutral-Density-Filter/dp/B00004ZCI2/ref=sr_1_1?ie=UTF8&qid=1418323615&sr=8-1&keywords=tiffen+77mm+graduated+neutral+density+filter+0.6 Sometimes I wish I had the option to block out more than 3 stops of light.