Least intrusive ad type? lol in your dreams. You are entitled to your own opinions not your own facts. Even Jacob Nielsen commented on intext ads:
One of misery design’s most insidious recent examples is the idea of embedding links to advertising on the actual words of an article using a service like IntelliTxt. By sullying the very concept of navigation, such ads not only damage the user experience on the host site, they poison the well for all websites. Such links make users even less likely to navigate sites, and more likely to turn to trusted search engines to guide them to the next page.
It is as annoying as banners that float and cover most of the screen forcing you to click their close button or pages that blast noise, music and or those "congratulations you won" flash ads. If you have to force your content/ads on people then you are doing it wrong. There are much better ways to provide advertising on a site...annoying your potential customers is a certain way to loose traffic.
Good thing that even Google blocks Kontera ads in the same pages where Adsense are posted. If you use Adsense on your page, Kontera ads will be blocked.