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Yes they count, and yes they can help your website, but they must play a very small role in any SEO strategy. If you're thinking you can comment your way to #1, you'll be disappointed.
We typically comment on very relevant blog posts, but only when the blog posts are new (it's best to be the first or second comment on a page full of comments).
We also spend more time commenting on sites that send referral traffic to our clients...some good comments have generated hundreds of referral visitors, and that makes the ROI of our work even better. Not only do we get some incremental SEO value, but we also get some direct visits.
Considering how cheap and easy it is to comment effectively, I'm surprised that more SEO firms don't talk about it...perhaps it's because of the stigma associated with spam comments (which I'm not at all endorsing).
If you are just posting comments that will be bad for SEO. But good participation and feedback on different blog comments might help you in SEO.
Agreed with chris adams
We only comment on very relevant blog posts, but since we mainly deal with Defence related business, its not very common.
Your comments should be constructive with good feedback and not just 'i agree with so and so' with your links.
Link building should be about driving more traffic and not simply to enhance SERPS. 9/10 blogs implement a "no follow" on blog comments. With that said, blog comments won't hurt your SEO provide the comments do not appear to have spam. Also, it will also depend on the industry you are in.