I have been taking 1,000IU of Vitamin D for the past weeks, and I have not notice any difference, which will make sense. All the information I have found online suggest taking 1,000IU of Vitamin D for various of reasons, cancer prevention being the top reason, but they also stated that one will have take that amount for three years or so for any "decrease in various cancers". Vitamin D has been showed to prevent several types of cancer (i.e., colon cancer, etc) and other illnesses, but that is only after a long period use of at least 1,000IU of vitamin D.
You may not notice any changes, but your body is definitely feeling the changes
We may not always notice any changes when using vitamins, minerals, and herbs (probably more energy or whatever).
Hope that helps!