The ability for us as an agency to manage our inventory across multiple platforms - Facebook ads & Google Adwords allows my clients access to reach their target audience without having any other middle man in between them (ie me). I can't think of anything that we dislike at this time but have not yet had enough campaigns running long term to make generalisations about what works well or doesn’t work so would like more data points before making comments here! We use it primarily within ecommerce sites. It has allowed us to be able to see who are customers where they live/work etc which helps when targeting audiences further down via remarketing lists.