.us is country code Top Level Domain for United States of America. Google and other search engine localize content on .us website to US market.
.us domain extension will help your Business in US in the following ways.
.us will allow your website to have a superior search engine rank with US. This will help your business to targets audience in US better than on most of other Top Level Domains.
Businesses running on .us are likely to enjoy better credibility from customers in US than many of other Top Level Domains.
Demerits of using .us domain for your business in US
.us localizes your brand to United States, expansion to Canada or Mexico or UK will require investment in another Top Level Domain and building another brand for the new market.