It depends on the quality used, how much is used, and the usage habits of the user.
Smoking it is still bad for you because you're inhaling burning hot gas and smoke particles. That's never going to be good for you.
There does seem to be a correlation between marijuana use with early death, obesity, heart disease, and chronically low motivation. Whether this is caused by marijuana or not remains uncertain, it could just as well be that marijuana is a common choice of underachievers, and poor school achievement is correlated to low career advancement, ultimately correlating with poorer health. And while I have no first-hand experience with the culture, anecdotally, most of the people I knew who were regular users weren't exactly shining stars, even before they started smoking frequently. It could also be due to the diets of people who use it a lot, since "the munchies" can cause cravings for fatty food.
Then there's always the chance that whatever person grew decided to take some liberties with what he used as fertilizer and pesticide, though that's probably going to be a smaller issue as marijuana becomes legal and, as a result, commercialized and therefore subject to the regulations imposed on other growers.
Even though marijuana doesn't form chemical dependency in the same manner as other drugs, it still has a psychological addiction factor. That can cause problems.