Absolute waste of time. The average forum page has 200 to 400 links on it and no or minimal PR. Even 1/400th of PR6 pages would be nothing. These links don’t help you at all.
Mostly a waste of time. The average blog page has little PR. Remember that pages have PR, not sites. Even then, if the blog doesn’t nofollow comments, you’re still competing with tons of links on the page for a fraction of that PR. And you’ll never create enough links on your own to compete for such a competitive phrase.
Assuming you have some PR to throw around and don’t have too many other links on the pages you link from, that can help.
That’s good, not just for backlinks, but to attract people to your site.
This is a very tough phrase to compete for:
All your competitors are experienced webmasters that know about SEO.
Much of your competition has a 15 year head start on you.
The only way it’s gonna happen is to have a better, more popular forum. I’m not exaggerating about this one. You will not outcompete the backlinks of 15 year old entrenched forums with hundreds of thousands of members unless you become extremely popular yourself. No link building you do can compare to the number of people continually placing new links to forums with millions of posts.