Digital Point's Coop Advertising network is really nothing more than a large automated link exchange. Originally it was supposed to be an automated link exchange for webmaster forums running vBulletin software. How it works is you load code on your pages which shows links to other sites, and then those sites load code that shows links to your site. Its all done server side, all random, so to a visitor or search engine it looks like a hard coded link on your website.
Shawn from Digital Point approached me about being one of the inaugural members way back when. He explained the way it'd work is that ads would only show on the vbulletin pages for viewing a single post by itself, such as this page. Since viewers rarely if ever view such pages you wouldn't be showing yet another ad to the viewers, but search engines do view such pages and so everyone would get the benefit of additional backlinks.
The code assigns weight based on things like traffic and Google PageRank, you can also get weight from referring people. The higher your weight the more often your ad will be shown. Also, unlike a standard link exchange, the site you put the links on does not have to be the same site you promote with your links. This is an ingenious idea and does make the network more useful than had it been a standard exchange.
I signed up (it was and is free after all) and it worked well at first with the small group of less than 10 webmaster forums, however the network was then opened up to more or less any site running PHP. Imagine my surprise when checking my backlinks I saw links from places such as a fishing site. What does website publishing have to do with fishing? Nothing, so I took the ads down, and lost a few thousand unrelated backlinks.
We all have our own limits and definitions of what is black hat and what isn't, and this is one of those things where you'll have to decide for yourself. I personally would not run this on any site where a Google ban would seriously hurt me. However, Google has been erratic lately, some sites simply do not rank well no matter what you do. In these cases you might as well join the ad network as it's not like it'll hurt you in Google and ad network sites do quite well in Yahoo and MSN, even Google sometimes.
Due to all of the above, my advice is to use this network at your own risk. I've seen some amazing results produced with this ad network, but in many ways it reeks of spam, so you will have to decide if a Google ban is worth the risk.