It really depends on which symptoms you want it to address...
The reason why there aren't many medications designated for PTSD is because the various symptom categories reference other already-existing diagnoses, and everyone struggles with different aspects of it.
So, when choosing meds for PTSD you'll want to identify what you need the most help with. MMJ doesn't necessarily help with all of what PTSD is, but it seems useful for sleep problems, agitation/anxiety, and I'm sure other things. It's not very useful for depression and suicidality...unless, of course, the depression is a secondary result of sleep problems - and on, and on. You just need to be able to tease out what you want the help with.
MMJ isn't right for everyone, but for some people its the missing piece of the puzzle.