The fact that this is open source software which you can edit yourself makes me super happy! It's been helpful to be able do things without needing someone else involved with it or having an account at their website (which I don't have). Not many features are available out of box but there has recently come some updates making them more user friendly in terms off adding new parts etc so they're coming along nicely as far development goes i suppose! Try using other projects too just see if anything works better than what was written by PHYSICS JS itself - like maybe Bullet? We used Physcisjs mainly because we needed something fairly simple/easy & free /open sourced quickly up until now when our project became bigger then its scope would allow us time constraints due to deadlines ect So not sure how much use bullet will bring though once finished :) Great choice either way really good tool!!