I think the current version of NumptyPhysics is pretty good playable and safe to use. There are problems (crashes and 'hangs') reported on the edit menu and the selection of levels doesn't work.

IMHO there can be a problem if you leave the extras-devel repository enabled and use updates from it or install other software from it. That's why i only enable it to install one piece of software and then disable it as fast as i can