Having a .Co domain instead of .net or .com or other domain extension shouldn't matter on how well you rank in search engines.
I have a website using a .Co domain and it ranks highly in Google Search and Yahoo search and does well.
It ranks well because of the excellent content that I've put on the website and Google and Yahoo and other search engines love it.
As long as you create great content and a great website the domain shouldn't have a problem ranking in the search engines.