From the point of view of Google this type of sites are normally have high trust however there are 100000000 blog sites that have a free access and now days people use it without any control so don't build on these sites.
Any link will boost your SERP and help in SEO. Some links are powerful than other. I don't think .org backlinks receive a special treatment in Google... probably they don't. .edu backlinks might give you more Google love but there is no 100% proof about this.
.org is no more relevant than .com or .net. But I DO believe that .edu or .gov does receive some extra value from google.
What about links from sites that Google deems to be in a "bad neighborhood"? Do these links help or hurt you? Not all links are good for business, folks. Here'sn article about the bad neighborhood effect: http://www.webconfs.com/bad-neighborhood-article-13.php