It is something like God: it takes one to be a believer to experience it and be convinced of it.
In today's world though, the applicability of karma is limited only to the current life, since future lives are more a matter of speculation.
Please note that in the Buddhist philosophy, Dharma does not 'punish' or 'reward' anyone. The concepts of good and bad don't exist here. For example, gravity is one of the laws of the universe, and if you jumped off a building and killed yourself, we wouldn't say that gravity was punishing you.