Depends on the baby. It's somewhere nice for them to lay & look up at things. Also to start tummy time on. It's easy to leave a baby in the same place for ages so it makes a nice change for them to lay out on a mat. Max enjoyed his from early on, looking at the toys & before long he started reaching up for them.
I got my daughter one before she was born wasn't really interested in it until she was about two months old maybe even later so I'd say get one for Christmas even if he doesn't use it till after at least its there for when hes ready
Not at all they can use it from birth. We bought one before Morgan was born lol. With Max we got it a few weeks after he was born with some vouchers we were given.