Providing there are not many outbound links form the site and the site /post is relavent to your site it is a perfect match. If for example you see a site that has 200 comments do not waste your site in commenting, very little link juice will be passed to your site.
Yes.That's what I read in many SEO blogs.But I am not sure whether its useful for Google's SERP or ALL search engines.But sure it will improve PageRank.