I've made a simple quadrapod with rigid body 6 DOF joints - with joint limits so it's only a single DOF per joint (w/without limits doesn't effect end results - I will have limits in the end goal though).
However, I am getting crazy instability. The joints do, rougly, seem to be working - but they just shudder and jitter a lot.
I'm not putting much force on them here. I have set them as dynamic objects, with collision convex hulls (not that boxes made a difference). I have the box ticked that says it disables collisions between jointed objects.
I am using rigid body joints as I intend to move towards some sort of machine learning for gait development - this rules out armatures I believe. Each leg needs to be able to freely translate and rotate in the world coordinate frame - this would allow gait development on uneven terrain, and for the quadrapod to fall over!