As I understand it, very effective. But most of the research that correlates low vitamin D levels with various diseases is from observational studies, so there's always a question of whether a low vitamin D level is the causal problem or if it's more complicated, for example maybe lack of sunlight causes other effects in addition to low vitamin D levels and those other effects are the real problem rather than the low vitamin D levels.
IFAICT, most doctors will still prescribe supplements for low vitamin D though. It certainly doesn't seem to hurt, as long as the levels don't get too high as discussed in this article.
