i ski a lot of indoor so we have these up regularly, i've never hurt myself on it, only injury i saw on a bag was either kids carving off too hard and missing the bag or a friend of mine who landed bad, ejected and took a ski to the face.....
then again all of those could just as well happen on snow, i'd say generally speaking they are excellent, i learned my lincolns, back and front on it before throwing them on snow, also did a lot of future spins and got really good air 'feeling' on these things without ever having to be scared of the landing...
if you hit them an entire day you do feel it in your muscles though :) the landings don't hurt but they aren't super soft either ....