The PPA program isnt exactly a new model and is basically the same as most affiliate programs that only pay for purchases or signups. Unlike normal adsense ads that analyse the content of a web site to determine relevent ads to show on the site adwords PPA allows the publisher to choose which ads to show on their site.
For publishers that don't wish to trawl through directories of adverts there will be an option to let Google choose the highest paying relevant ads from various topic areas. Revenue earned will most likely be comparable to PPC revenues for sites though but earned through a low volume of sales as compared to a higher volume of clicks.
It impossible top predict what will happen but it certainly will be interesting to see how things pan out. Google is a pretty savvy operator in this field so will probably have its plans well founded and make their latest product a big player in this area very quickly. They will need to ensure that the publishers can earn as much or more than the PPC network using PPA though or no publishers will sign up for the program. If there are no publishers , and remember this relies purely on publishers as it is on the content network only, then there will be no advertisers.
Google will surely realize this and most experts expect when the program comes out of beta then there will be plenty of incentives for publishers to sign up.
For publishers that dont put the work in with the new program and choose adverts that aren’t very relevant for their readers then they will most likely not have much success with PPA.Those that know their markets or have very niche focused sites could make good profits. Expect to see lots of new sites starting up to focus purely on google PPA topic areas also.
More info on googles PPA program can be found here : Google Pay Per Action