Humm.....
So lets assume that you are talking about searching blogs related to our theme of site and then make a good comment on that blog.
Now all that is included in this, is to find a relevant blog and then make a comment on it. You will also be posting same comments on multiple blogs. Right.
Now there is a software and a script for this, which finds blogs related to our given keyword using google API key and posts comments atleast 100 comments every 1 minute. [And this now becomes SPAM]
Plus If I find the same sites from google.com manually and post the comments to them manually and waste 10 hours in posting that 100 comments then half of the people will say it is Not called SPAM. heheehe why because I wasted time doing the same thing that a software could have done easily.
If this is a SPAM technique or not that is an issue which can cover at least 100 pages if a new thread is started about it.
But to see the results I had done this kind of thing in past with one .info domain that I regged for 0.99 :D
And the PR is 1 right now.

So not much success with it.
On the other hand I told one friend of mine to build some links for a website which was also started in same time and with similar theme and content. [Similar not same]
And the directory submissions brought PR 4 for that domain name.
Sometimes this technique works for some sites. EX: Another forum which I was doing SEO for got very good weight in SE's due to the blog posting. I was just passing time on DP and was exchanging blog posts with other webmasters and nothing was done not a single backlink except the blog posts and the forum kick-started itself in 1 month.
So now lets see what other people have got in the results.
My personal experience that I would like to share as this thread is for sharing the experiences based on this technique:
I have already done this and the results are not same for all the sites.
You might get lucky in some cases. But if you are really experimenting on this one then try it on some useless domain and stop any other kind of marketing to see the perfect results. Because forum links, directory links might be bringing PR and you might be thinking that blog posting brought it. On the other hand blog posting might be bringing PR and you might think that your forum posting brought the PR.