okay,I want to register a domain xxxx.com ,but it was already taken,and xxxx.us is available,so it worth to register?and what's the disadvantages of a .us domain?
The disadvantage is that it doesn't geocode properly to the US, like other ccTLDs geocode to their own countries. I've never seen these domains get preferential placement here. Think about how many .us's you've ever seen in a search result. I bought a few the day .us went public and have developed some very content-rich and heavily promoted sites on them, and they have hit glass ceilings in terms of ranking for anything better than some 3 or 4 word keywords.
Many settle for the ".com" extension and no less. As grounds for their arguments, they cite the browser algorithms used to locate a website when a user simply types a name like "acme" into the browser. Apparently, the browser searches for a domain name "acme.com" before attempting "acme.net", etc. As such, people who do that will be delivered to your competitor's site if you do not also own the ".com" extension. Indeed, even if people do not rely on their browser to complete their typing, many simply assume a ".com" extension when they type a domain name, so if your business is "Acme", they'll just assume your domain name is "acme.com" rather than "acme.net" or some other such name.