Welcome to WebmasterWorld Guest from 188.8.131.52
See sample here [img20.imageshack.us]
This is enabled on all ad format, 350x250, 200x200, 160, 120, 480 etc. Looks quite good.
The people that ignore the ads will continue to but maybe a few will be intrigued and clicked the arrows. That means more impressions and possibly an untapped market for clics
The people that already click the ads will more likely click those arrows and browse and click. That means more impressions and more clicks.
ad targets my sites well, and I am careful to post ads only on pages where they might serve the reader.
As Breto posted already mentioned, this feature on pages with really relevant content serving the needs of the readers, will do very well especially where persons are actively looking for information to take action. Even if the leave your site they will be back because you pointed them in the direction they wanted to go.
As for warning of changes. That is true but I can bet it is covered in the fine print of their terms and conditions.