I have found any benefits of adding subdomains to an established site to be short lived at best. However I have have success with a site that was created with subdomains from day one. The theme is cityname.domain. The fact that it is using citynames may or may not be the reason. All other subdomains on my other sites have tended to see an intial bounce followed by a downward spiral. In one case I used a 301 from subdomain back to same content on main site and subs went supplemental and the content on main site recovered traffic quite nicely.