The open source platform and community are the best features. Everyone is willing to help and share ideas and knowledge learned over the years.

Help documentation of OpenSim is well described.

I have used OpenSim for many applications. I was successfully able to integrate the real-time motion with open sim models. That is pretty cool and it helped many people to detect some unusual behaviors. Apart from that, I have used cerebral palsy children's model, tendon transfer surgery simulation, and etc as teaching materials. I am still using it for my research purposes.