I used it in my old 29gal's refugium (part of the sump, about 15gals). Honestly, I'd say it's nearly impossible to tell if any benefits, or any problems for that matter, are the result of adding Miracle Mud.
I was able to keep a couple things that are notoriously hard to keep, tree sponges, non-photosynthetic gorgonians, large feather dusters, alive longer then average in that tank. Again, I have no idea if it's a result of the "nutrient rich" Miracle Mud.
However recently there's an article somewhere online that more or less debunks Miracle Mud. They basically find that the stuff is most likely not even from the ocean. I'll try looking for it later.
If you still plan on using it, I definitely wouldn't place it just in the sump, only in a dedicated refugium. It's quite dirty and can very easily cloud the water.