As a three cat household we use Feliway all the time. It calms down the aggressive behaviours of the boys, stops them spraying inside and just generally keeps the them in a more settled mood. It's a great product, would recommend 10/10.
My cat Archie, was not happy with a new addition to the family and was very stressed, Feliway has helped him calm down a bit and he is at least now tolerant of the new kitten. When we have stopped using Feliway he has reverted back to being one angry man, so it definitely works.