From my experience posting inside the forums can be useful only if you make a lot of posts because the backlink ratio for this pratice help only a little, anyway i don't feel that pr is the most important think, I had a website banned from google for 1 year without an apparent reason, and this site was the most visited of my network, I received visits from the other search engines and directories and they had more relevance than the actual google searches that i receive now.

Forums are good for generating targeted traffic and strengthen your online presence and it also help to build backlinks but it all depends upon the PR of particular pages where you post, generally the internal pages have very low PR or even few have "0" PR, hence obviously thats not going to be considered as good backlinks.

why are my link:s not coming up in google?