You have to realize that if you register cars.us.com, that "cars" is a subdomain of us.com. Be careful as if the owners of us.com ever sold out to anyone, potentially all subdomains would be lost - potentially. This has happened before in the past, however, since all registrars are starting to sell these subdomains, im sure there is *some* sort of written agreement in place, but I could be wrong.
There has been much debate to this in the SEO community. Some people view it as horrible, and others realize it is a subdomain. Many people think subdomains can rank just as high as .com's. So if you wanted to rank for cars, you don't necessarily have to get cars.us.com you could just make cars.YourNormalRegularDomain.com.
Realize though that it is a subdomain, and will not rank the same as cars.us, as .us is the united states.
Over all, its worth a shot, but be careful. Many great names are still available for a reason.
Thanks for the answer, "domain-i-want" is available only under .us.com, anything else is taken , from .nl to .cz or .si . I decided not to buy it. As you said, i can't trust the owners of us.com.
Probably a bad investment. If I was running a seriously business I would never operate under a subdomain controlled by someone else. At most I would use such domains for mini-sites generating traffic to my main site.