I did some research on it before I put DS in it, and from what I found, a lot of the dangers could be significantly reduced by setting the height so that they were bouncing flat footed rather than on tippy toes, and by limiting time in it.

I was a little concerned that JJ's can place the hips in a position that is not "normal" for their stage of development, but my MCHN (who is, oddly, very concerned about hips) did not seem to think this was an issue.

I'm surprised to hear about backs. To me, it seems like what he is doing in the JJ is pretty much the same thing he is doing when I hold him upright- ie- bouncing like crazy- but without killing my arms! I suppose the different might be how long you do it for, and, as we've all said, foot position.