I'm sure you can find tons of info if you search... But from what I gather, reciprocal linking is better then no linking, and one way links are better then that... But most important, make sure the linking site is related to yours...
Having related theme sites link to you has no effect with ranking. Google gives weight to sites that they believe have authority in their niche and will reward any website that external links are directed at. For example a link on an authority golf website to a car hire company will have weight because google will see it as a credible link thus allow it an rank it accordingly. However, if you have a link on a high traffic golf website for a car hire company you will gain natural visitors from golfing users looking for car hire companies when golf holidaying, It can be very complicated but getting links on authority(high PR)websites is all good.
So your saying, if I link my PR4 adult site, to my PR1 programming site, it's the same as if the PR4 was another programming site? I don't believe that... And if so, how do people know this? Is there someone in Google that leaks this information?
Google won't penalize reciprocal linking unless you go overboard with it. But they will devalue the link. However, linking to an adult site is not a good idea but linking from an adult site should not be a problem. Yes, and if that authority website is from a related theme it is even better.
Absolutely agree with you. I made 118$ in 24 hrs. How? My link http://mystanding.blogspot.com/2007/10/amazing-lady.html has been added to an adult site. Well after 24hrs. Google got me and started displaying social ads. No ban, no warning emails, no penalty.
Of course one way links are the best one's and if they are quality, relative, loaded with PR and are not paid ones then nothing like it in terms of organic link building process.