Google AdWords - The biggest and best for volume, traffic quality and user interface. However the most expensive because of those qualities. Yahoo! Search Marketing - Both high volume and quality. 7Search - A smaller search engine with good quality traffic. So if you want quantity you might have to look elsewhere. MSN AdCenter - Very high quality traffic MIVA - This is an underated PPC search engines but has good quality traffic. Hope that has helped
in terms of volume though, is Google adwords volume an order of magnitude larger than the others? it is even worth bothering with the others, given they have such a large share of the search market?
The same I can say. It's a great experience. But now I would like to work with another PPC to compare...