IMO dofollow blog links sucks, unless they are on a PR7+ blog. But here everybody is mad to get as much as he can. I don't think it worth so much - check a PR4 blog, comment page is PR N/A, with 376527 piece of outgoing links. The link in my signature here worth more...
If our comment gets approved on high pr blogs then we can get backlink on it. And more backlinks means more PR
because its a one of the best Technique in SEO and there is much better chances to get Pagerank Juice, back links and Traffic. these are the chances to submit comments on dofollow. i think, you will try to submit your Helpful + Quality comments on blog ? Thanks John
IMO 1000 commentlink in underPR6 blogs worth around $10... (equal to buy a blog post to a 250 blogs network for $10) And of course the finest if the blogmaster click on an potion box, and from that your comment become nofollow. Commentlinks sucks as well as directory links...
It helps to gain PR and traffic to your site at the same time. And it is an easy process if you have the right list. As 90% of blogs are no-follow. So it is not good for link juice. Get your self a good list of high PR do-follow blogs and do the comments to get backlinks, it is what "normal" SEO company do anyway.
Sure, it is always better to leave comments on do-follow blogs, but from my understanding even no-follow links give you a little credit. In other words, a no-follow link counts as half or quarter of a link (or something like that). So, not as much link love, but still some.
Comment links don't have much of worth. And you can't of course use your keywords while commenting, you got to put your name in. So, it doesn't help at all. Many people comment on dofollow blogs just to up the number of backlinks. Its something a helpless SEO master will do as a last resort.
A nofollow link will do equally good to generate traffic. Hardly anyone posts a comment on dofollow blog with purpose of traffic.