allmost all of us start promoting own website by blog commenting. But to be successfull u need to have a blog-list and nobody share it. I have a programm FastBlogFinder3 - it helps me well
Depends on your comment. I get plenty of click throughs and a few purchases from posting legit, thoughtful blog comments on other blogs in my niche. Using your keyword as your name might be best for SEO, but adding your first name seems to help click through.
it is still best to comment on related blogs. make your comment relevant to the post. try to add your link in the comment or add your signature if they allow. don't worry about no follow -too many of them won't help your rank anyway. help the people and they will help you. and stop spamming blogs - those comments get blocked anyway
With respect to commenting on Do-Follow blogs, they still won't help your PR very much, even if you're commenting on a high PR blog that is relevant or irrelevant. This is because the blog page you're commenting on has a much lower PR than the entire blog itself. That is, if you're looking at a blog with a PR7 value, you'll find that the individual blog page you're commenting on only has a PR1 value, if that. To answer your question about comment links from an irrelevant website, it certainly will increase your PR. All inbound links will increase your PR. It's just better to get inbound links from related websites.
Yes.. offcourse your PR and Rankings will increase.. but do not do spam.. and target more then one keyword..
Try to keep it relevant. Also, it'll help if you see past the instant gratification of a quick link. Provide value, see if you can build up a relationship with the author of the blog. That's far more valuable than a spammy link that might end up getting deleted anyway.
Blog commenting is help to get high page rank. Blog commenting helps to generate more back links which are used to increase traffic.