In reply to toasted:
I got one, never tried the exact exercise but tried let raises etc and was fine, obviously need to be controlled which to be honest leads to better form than just kicking up with no control.
I tried (almost) inverting and touching the door frame with my feet on another bar (not a powerbar but similar idea, that one actually came with fittings you could screw above the door but I didn't bother). Again I was really controlled and balanced though.
I guess it also depends a bit in the width/strength of your door frame etc but I think if you stay balanced and move slowly, I think it'll probably be fine but personally I got fairly nervous hanging upside down from it, had i come down I am sure it would have been a pretty serious injury on a hard floor (i am sure i wouldn't stand a chance of reacting so i'd land on my head - ouch) so I'd leave them for the gym if I were you - especially since I think the biggest risk would be on bringing your legs back through...
Post edited at 23:22