From an SEO and Internet marketing viewpoint, leaving comments on do-follow blogs offers the following benefits for : 1. Get back links to your website. 2. Get link juice and page rank back to your website. 3. Get traffic to your website. 4. Improved search engine rankings 5. Get more visitors and leads to your website. 6. Get maximum SEO impact for participating in discussions. 7. More importantly, you will get long-lasting results using 100% Google-friendly tactics.
but the things is you do not get the instant approval do follow blogs and the other thing is that some never approve your comment if u have used any keyword in ur title
This doesn't mean you should avoid no-follow, building do-follow and no-follow links seems more natural to the SE
blog commenting can only prove to be helpful if the blog post allows do follow commenting. But in some cases like if you are commenting on popular blogs like seomoz, searchengineland etc. then even for nofollow commenting you can have traffic through clickthrough.