Disclaimer: one of the marketing managers at SwitchAds approached me about testing their ad service.
The concept of this advertising service is a good one. While advertising services such as Google AdSense can usually fill two or three ad blocks on your web page regardless of the space available and are the best priced ads of those sizes that Google have available at the time, they are not necessarily the best priced ads you can get to fill those ad spaces on your site. The SwitchAds concept is one where SwitchAds attempts to sell the advertising space on your web pages at a higher price than your current ad service (such as AdSense) is selling the space for. When they are successful at selling the space they automatically switch in the higher priced ad in place of your default ad service - hence the name SwitchAds. This should give you the best of both worlds - higher priced ads where SwitchAds is able to obtain them for you and ads from your default ad service where SwitchAds is unable to find a better price or where the ad space is not one of the SwitchAds standard sizes (such as on handheld devices where only small ads will fit).
When signing up you are asked to nominate a web site that you want the ads to display on (there is an opportunity to add additional web sites once you finish signing up).
The manager who had approached me in the first place had asked me to contact him once I had signed up - which I did. I also advised him of the AdSense code that I am using on each of my sites (which is identical across all of the pages on a site and which differs only in the slot numbers between this site and the other sites where I use it. I got no further response from the marketing manager.
A couple of days later having received no response from the marketing manager who first approached me and having received a welcome email from an account manager when I first signed up, I decided to ask the account manager how to get the code to set up their ad service.
A couple of days later having still received no response from either the marketing manager or the account manager, I decided to make one last attempt at getting this ad service set up and raised a support request asking for help. I got a reply back the following day stating that to set up ads I needed to fill out the ad block form on their site for each ad block I wanted to use with their service and that their code makes no provision for testing if the selected spot is big enough to display the ad or not. The suggestion was that I fill out the form and that if they had an ad available to fill the ad slot it would display at the predefined fixed size (whether it would actually fit or not and that the AdSense fallback should responsively fill the slot with an ad that does fit if they didn't have an ad to display there.