Interest groups on the content network have been available for some time, but I've
spoken to a number of people that either haven't tried them, or tried them once, but abandoned them due to lack of performance. Most often, their idea of 'trying' interest groups meant creating an ad group and assigning an interest audience alone to them. But that's like deciding to buy a commercial on Spike TV just because your primary audience is guys. To make the interest audience work, you have to take things a step further.
Keeping with the analogy, say you sell Jeep Lift Kits on your eCommerce store. You'll want to create a separate ad group in your content campaign (because you never mingle content and search in the same campaign),and assign your desired interest group to that ad group, for instance, Off-Road Vehicles. Cool, now your ads will show to people interested in Off-Road Vehicles, on potentially any site, no matter the topic.
That's a horrible idea! That's like putting a Jeep lift commercial on Spike TV without specifying the program in which you want your commercial to run. No! Of course you would want your commercial to run during the Power Block on Saturday mornings. Or even more specifically, during the show '4x4' because that is when viewers are engaged with Jeeps and other off road vehicles. Higher engagement with the topic means better results, every time.
So how do you 'choose the TV show' in AdWords? You cross reference your ad group with relevant keywords. By doing this, you are effectively deciding on what topics of web pages your Jeep Lift Kit ads will be displayed. Now you have an ad, being shown to someone who has demonstrated an interest in Off-Road Vehicles, on pages that are related to Jeeps, thus your engaged audience.
Another way this method improves results is, it helps verify the visitor. My wife and I have different computers, but occasionally I'll use hers. The websites she visits, compared to the ones I do varies dramatically, so that means our computers are tagged as having different interests. If I happen to be on her computer, I could potentially be served ads about 'Pretty Little Liars' while I'm browsing a popup camper forum. Conversely, she may get backpacking ads if she's on my computer browsing about the latest 'Twilight' gossip (if there is such a thing). With keyword qualifiers, her ads won't show to me (unless I too go to the gossip boards) and mine won't show to her. Many families have one computer in the household, so think about how many different interests are identified on those!
Flip the coin. Do you have a keyword contextual campaign that is under-performing? Narrow your audience by adding an interest group to that ad group. That keeps your ads limited to relevant pages, to people who have a proven propensity to be interested in your topic (or one that is synergistic).
So give interest audiences another chance, this time with a little backup, and see what happens for you.