I've had a medical card for a little bit now, and I find it to be a big help. Great for my anxiety, insomnia, pain, muscle tension, and appetite. It can slow racing thoughts and, in higher doses, help reduce unwanted thoughts. If I have a bad emotional flashback, it can help me become calm and rational again... but if I'm still around the trigger, it's useless. It's also supposed to be amazing for PTSD nightmares, and is being studied for that in the US now, but nightmares aren't common for me right now.
There are a number of modern ways to consume cannabis. I don't think most medical users smoke it—at least not in my area. You can even get it in capsule form! In vapor form, it's a take-as-needed medicine with very fast onset, so I feel some of the anxiety melt within seconds. For me, side effects are minimal. As
@DeathRay said, there are many different kinds (strains) of cannabis. If your symptoms aren't helped or if the side effects are unpleasant, it's likely that a different strain will be helpful.
There are a few negatives. Just like alcohol, food, or video games, it can be misused. However, I believe the majority of medical sources consider it much less harmful and much less addictive than alcohol and nicotine. Some sources even state it is less addictive than caffeine, oddly enough.
At the end of the day, cannabis is not a cure for PTSD. It just helps manage symptoms.