I got one when DS was 6m old; I still use it at my Mum's house. Because she has a big house! I don't use it at home, we can hear DS when he's supposed to be asleep. His cot is against the wall our bedhead is against so he wakes us up easily.

I did use it at home when DS was about 10-15m old; I was in a different room and it gave me peace of mind. Then DS slept through for a bit, so we stopped using it. Then when he stopped sleeping well we never bothered starting it again as we didn't need it.