Not so useful when they are tiny and stay put (unless you are a nervous parent and like to see them), however great when they are older and you can see if the noises are them dropping a book out of a bed whilst settling/flailing about and hitting the wall if they move about a lot in their sleep - no action needed, or if it's due to them being out of bed and up to mischief meaning one of us would have to go up.
We used our for longer than was strictly necessary because we had two in the same room at a similar age who liked to get out of bed and play when they should have been settling. It helps not having to go up for every single noise. It was also cute to see them do things like "read" each other stories before going to sleep.
Ours DDs also worked out that we could see them and used to talk to us over it (better than shouting down the stairs). Obviously we would go up to them if needed but sometimes it was helpful to avoid going up for every little thing and avoiding reinforcing that being disruptive gets extra attention from mummy or daddy.
It never felt creepy for us and our DDs seemed to find it reassuring.