Yep as everyone else has said they are safe for short amounts of time, you also need to make sure your LO is in it correctly and it's the right height.
There was some concern a while back about it being bad for baby's backs but I think it was a case of people not following the instructions. I would recommend getting one of the newer ones as they have changed the design a bit to make it safer.
Hope that makes sense. Cant think straight today.