Google is the market leader as far as search engine traffic in the United States is concerned. Bing and Yahoo Gemini share the remaining 20% between the two of them (source: Searchenginejournal.com). The first priority should be well designed and optimized Google Adwords campaigns but later on Bing and Yahoo Gemini should also be added to the mix. While Bing and Yahoo Gemini offer to import your Google campaigns automatically this is not recommended. Campaigns can be imported as a starting point but then they need a lot of work and tweaking before you can expect a positive Return on Investment (ROI).