It's easy to use, it has all you need for display ads without much hassle or additional costs required from your side! The tool is super flexible but still very intuitive (I'm using them since 2015) so I can't find many things that should be improved in my opinion at this moment :).
If there are some issues with an ad format which aren´t supported yet by their platform then they will resolve these asap if needed :) We're running our own account here where we serve different types like video rich banners etc., just perfect!