I think that all drugs have a negative impact somehow. I think there are benefits to some drugs that can outweigh the bad, and I also think some drugs aren't as bad as alcohol. I think marijuana has positives that outweigh the negative. The single negative being smoke is bad for you. If you aren't out driving while high you should be good.
Marijuana helps people with headaches, insomnia, etc. It leaves you well rested and feeling fresh and ready for the next day. It has been proven to do many other medically beneficial things as well. One major thing that hits close to home for me is it helps prevent seizures. If you've ever been in the room while someone has had a seizure you'd understand. It's scary. As long as you use the drug for good reasons. Example: Help you sleep, enhances your eating experience, relieves you of headaches, makes you feel better the next morning, I don't see a problem with it. I think some people do marijuana to get messed up and can be reckless by their own choice but most just do it to help them wind down and enjoy things to their fullest.