Hi Colette,
We always had a pull up bar when we were kids, so a few years ago now, I picked up a cheap one for my son. Let's see...my husband has knocked his head on it a few times but not recently... the only time that it has ever come down is when the kids were jumping and swinging on it and of course that was after lots of warnings not too...we have a very old house so the door frame has held up very well but my mother has a new house and she had to get the jam braced on either side of the bar, otherwise it would most definitely have bent the frame...I love it because not only can you do pull ups but you can work your abs real hard by hanging from it and raising your legs as high as you can and doing as many reps as you can...with your legs straight or bent it is tough!
Just my 2cents,
Jodi