Blog commenting may help you a little in traffic. Most blogs use nofollow tag for comment link, and readers often never click on comment links, except there's a catchy message Personally, I rarely read/click on comments
Yes, Do follow blogs will drive both traffic and link juice whereas nofollow will just drive traffic for your website
It depends on a few things and which engine your talking about... I generally focus on getting URLs to rank at Google and then just take what I get at the other engines since MOST of the web's search traffic originates at Google. So as far as ranking at Google is concerned... As long as the blog allows FOLLOWed comments then you should at a minimum get a little Page Rank (PR) from the comment link. But this alone is not likely to help you rank since we all know that PR plays a miniscule role in Google's ranking algorithm. To assist you in ranking at Google you will want to comment on blogs relevant to the page on your site that you are promoting and farm backlinks to relevant pages on your site with link text that contains keywords that your want those pages to rank for. Blog Commenting Tips The first key to successful blog commenting IMO is becoming trusted by the blog owner to participate and add to the conversion or discussion about the post. Don't just post comments like "nice post". Post a few paragraphs... maybe give your own personal examples of what they are posting about like "What you're saying about <insert blog post topic> is soooo true. Just the other day I encountered a similar situation... <blah blah blah>." This is VERY important because most do-FOLLOW blogs are moderated and your first few posts are going to determine whether they accept your comments or not. You need to be perceived by the blog moderator as adding value to the discussion, not just spamming for links. When you enter your comment you typically have to enter a name, email, and URL. The second key thing about blog commenting IMO is getting away with entering a name that has keywords (or a keyword phrase) that you're trying to rank for as well as a URL pointing to the page that you want that keyword or keyword phrase to rank for. When picking a name it's hard to get accepted if it's obvious you are using keyword phrases you want to rank for. So I typically add "Man" or "Guru" or something like that to the end of the phrase I'm trying to rank for. For example, if I want to rank for "mortgage calculator" and I simply enter that as my name, I seem to be denied more than if I enter "Mortgage Calculator Guru" or "Mortgage Calculator Man" as my name. Sure "Mortgage Calculator" would be better because it maximizes the keyword density within the link text and is the EXACT phrase I want to rank for, but if it causes my comments to be denied most of the time then what good has it done me. So I would use a slight variation of my targeted keyword phrase instead... something like "Mortgage Calculator Guru". Not quite as good as "Mortgage Calculator"... but it's still a very good link text containing my keyword phrase which is at the very beginning of the link text. This type of link text seems to lead to your posts being accepted much more frequently. Your home page URL is typically the easiest URL to get accepted. Many sites wont accept your comments if you're posting to an inner page because it's a sign that you are an SEO or spammer farming links, although I always try this first. If after a few comments using an inner page URL they never show up then I will take a short break from that blog and come back in a couple of weeks to start trying to use my home page URL. Something else I've found that seems to be important in adding to your credibility as a commentor... The email address that you enter should use the same domain as the domain in the URL you enter. If you are blog commenting on behalf of other sites that you do not own because you are doing link development for them then I highly suggest that you have them set you up an email account on their domain prior to starting you blog commenting campaign. There are simply things I've observed when blog commenting. Perhaps others' experiences are different. I admit I don't do a whole lot of it because I tend to go after quality links rather than a lot of low quality links... but I have been doing more and more of it lately for pages where I can't find a lot of good links. Hope that helps...