I built a site related to food product review and posted around 50 blog comments on various blogs.
The site was at 4th page before blog commenting was done. I just checked today and VOILA! the ranking has improved.
Now my site is on 2nd page. Yeah, i know its nothing great but at least major improvement is there.
Now i will quickly go and write some articles and hope that it gets on first page!
PS: i only did blog commenting for this site and no other SEO activity.