John Battelle has a writeup about why some AdWords ads work while others don’t (and it extends past AdWords and into PointRoll and other ad technologies).

When users DO look at ads with graphics, those ads usually have:

• Heavy use of large, clear text
• A color scheme that matches the site’s style
• Attention-grabbing proprieties such as black text on a white background, words such as "free" and interactive (UI) elements.

I saw stats on one of my clients recently that suggested PointRoll ads don’t do that well for conversions.   I’m sure PointRoll could show me stats that prove their ads do well.  The point is - finding what works for each client and overall.