Well, it depends on the context. If you would like to rank for a competitive keyword, blog commenting alone won’t be sufficient to get a Google top 10 ranking. On the other hand, if you try to rank for a keyword with relatively low competition, commenting on 10-15 dofollow blogs could be enough to get you into the top 5.
But, of course, it also depends on the overall quality of your site (on the trust rank you accumulated over time). Getting a top ranking for a low competition keyword on a quality site may take only 1 or 2 comments on other blogs. On the other hand, getting a top ranking for the same keyword on a poor website with no (or almost no) content, with low trust rank… will require much more effort.
All in all I’d say, blog commenting (and especially dofolow blog commenting) should be part of your SEO strategy. It is relatively easy, it doesn’t take much time and you can get some great backlinks from high quality sites.
I use blog commenting to build backlinks for my sniper sites as well as for my long-term and content rich websites. It is not some sort of magical solution to get #1 on Google (anyway, this kind of magical solution doesn’t exist), but it definitely helps to improve your rankings. Also, it is always a good idea to build backlinks from various different sources.
To find tons of great blogs for commenting and for building backlinks, I recommend you download my free blog search engine software from the Free SEO Tools section.
You can find a detailed description of the tool as well as a screenshot and a short instructional video on the following page: Free Blog Search Engine
So, yes, I recommend you make blog commenting a part of your SEO strategy. Just make sure to post valuable comments. There is no need to write a novel. 1 or 2 sentences may be enough. Just provide a useful comment and avoid those: “Thanks for sharing, great post…” comments.
Also, when you find a blog post and there are already lots of spam comments on it, just leave and look for another blog. A backlink from those kinds of pages is absolutely worthless in the eyes of the search engines -even if it is a .edu or .gov domain, so don’t waste your time.
In my opinion, nofollow backlinks from quality sites are also beneficial and they definitely can help to improve the rankings of your website. I prefer a nofollow backlink from a high quality website, rather than a dofollow backlink from a low quality blog with hundreds of spam comments.
Also, your backlink profile should look as natural as possible and having only dofollow backlinks will surely raise some rd flags with the search engines.
For your blog commenting, I recommend you get a good mix of different types of backlinks. Get some dofollow as well as nofollow backlinks. Comment on standard WordPress blogs, comment on some .edu and .gov blogs.
Find some blogs that use the KeywordLuv plugin and that allow you to use a specific keyword in the name field. Comment on blogs that heva the CommentLuv plugin installed – that way you can get a second backlink (from your latest blog post) from your blog comment.