According to cnvi:
That's about true. Here is what Google's site actually says:
And here [google.com]:
And over here [google.com]:
So what that means is that unnatural links are pretty much everything else (outside of expert directories):
I am being honest: We are here to discuss the creation of both natural and unnatural links (and to get away with the unnatural links). I don't claim that reciprocal linking is ethical or unethical. I'm only pointing out that Google's own guidelines do not endorse reciprocal linking, and that Google's own guidelines are explicit that anything that is not naturally occurring* because of your great content is outside of their guidelines.
Now I ask you, how reasonable is it to refer to reciprocal links as a branding and traffic project? That's like telling the judge you were marketing marijuana for the pleasure of it's taste, and not for getting high (because that would be unethical, right?), so therefore your marijuana sales are not illegal. That claim would hold more water IF they removed the THC from the marijuana (or added a no-follow tag to the links).
If you as a webmaster were doing reciprocal links as a non-SEO, NON-pagerank influencing activity for branding and traffic trading purposes only, then you should have no qualms with asking for a NO-FOLLOW tag to your incoming links, and adding them to your outgoing links.
But I think most webmasters prefer to get high... in the serps. ;)
*Expert directories like DMOZ are the exception, as the guidelines explicitly endorse them. Likely because they are hand edited for quality.
Is there value in reciprocal links? If it's a high quality site, there's probably value in that. How do you judge quality? Ask yourself, if you were a DMOZ editor, would you add that site to the directory?