S.,
Here's my best answer, used it for two kids and plan to again with our third.
1. Exersaucer - the kind that moves, but does have breaks on it. No risks of falling down the stairs in our house.
2. Used it when the child could sit upright without any risk of banging face on "table". (Don't recall exact age, more a developmental stage.)
3. Wanted to have a contained way to help baby sit, propped up and strengthen legs, say while I cook dinner.
4. I don't think it harmed my children, I think it helped with leg strength. However, I do know people who think it hurt their kids. I was told by a nurse-relative to ensure there were no risks of falling down the stairs, and to always make sure that the height for the seat was set so the child's feet can touch the floor flat. That due to the femoral artery, you don't want to cut off circulation. Also, that if the seat is not low enough it teaches the kids to walk tip-toed instead of starting to learn how to support themselves using their whole foot.
5. Again, don't have a firm # here since it was several years ago, however I'd say no more than 30 mins a day.
6. Don't recall Ped. having an opinion. Like I said, I read/heard the pros and cons (as listed above). We knew them and always kept our adjustable seat low for our kids. We only moved it up after they clearly could stand and sit with both feet firmly on the floor.
We never owned a Johnny Jump Up.
I hope this helps.
Sara