AdButler is very convenient and very cost effective. Additionally, when you change a setup at the ad level, the server shows the change immediately; there's no delay.
Ad Manager is better when you manage a significant amount of campaigns and users, but it's not that cheap in …
AdButler is suitable for any company size but I would said that it is better for small to medium-sized agencies. AdButler is highly scalable but the control panel/dashboard could be challenging to manage if you have many campaigns or many users. It's also a good alternative for ad networks, which would like to manage multiple relationships with publishers.
AdButler is very convenient and very cost effective. Additionally, when you change a setup at the ad level, the server shows the change immediately; there's no delay. Ad Manager is better when you manage a significant amount of campaigns and users, but it's not that cheap in the end and you always have to wait for any change. EPOM is also a good solution, but the look and feel are old and it's complicated for some users to manage properly.