I have a client considering using a .us domain name. Are there any particular reasons, other than familiarity, not to use a .us name? In this case, the site visitors will likely be mostly non-us based - is it possible that this could mitigate any familiarity issues (with more international users being used to seeing non-.com domains)? Are there any SEO reasons not to use a .us name?
It terms of public perception I think the domain is becoming less and less relevant. There are (at least) two factors at play here.
Firstly, I'm seeing more and more adverts for companies that simply say "search [terms]" when the company wants to direct you to a web site. This means that the user isn't typing the site name and so is less likely to notice what it actually is. Witness people trying to log into a blog about Facebook thinking it was facebook.
Secondly, when domains are mentioned they are often vanity URLs such as "five.tv" (a television channel here in the UK) rather than "channel5.co.uk".
Thirdly, there has been an expansion of TLDs recently.
As for the SEO side of things, I would have thought that it's the content and incoming links that are important rather than the domain - but I'm willing to be proved wrong on that.
See this Question & Answer for how the TLD affects SEO.
If the site's visitors are going to be international I don't see why your client would choose a .us domain name. Although the meaning of TLDs does seem to be less meaningful then in the past it still does have some meaning. It's likely that some users will see the .us TLD and be confused or concerned (depending on the content). Just doing it for SEO purposes would be neglecting the user which is always a bad idea.
I own a environmental company and we just switched to .us as our domain my business has increased 3 fold since the new domain change.The .US domain is not as restrictive as expected. It is intended for all US related entities, including not only United States citizens, residents, or organizations, but also foreign entities presented in the United States. The domain became available in 2002 when all second-level domains were released for public registration, adding more options to the available top-level TLDs such as .COM, .NET, .ORG, .INFO, etc. Before its official release, the .US domain was primarily used by governmental agencies. After the release was a fact, that tradition was preserved to some extent, so that many registrants in the USA preferred the .com, .net or .org extensions to .us. That trend has been interrupted in recent years when a growing number of US related individuals, organizations or businesses resort first to the .US extension for their preferred domain.