of you do blog commenting on do follow site then definitely your SERP will be increased but if you do blog commenting on no follow site then you will get good amount of traffic
It's very useful but it's gets tiring after a few months. I now write press releases and market my site offline and it does well.
Sites with do-follow enabled in comments will give your good SERP, otherwise its of no use. Submitting your pages to bookmarking sites can also increase your SERP. PM me if you need any assistance...