There is no "link value". Maybe there is in Yahoo and Bing, but not Google. But as Bill stated, the traffic value can be substantial if your blog comments have value.
I usually leave long blog comments and sometimes I create more targeted traffic for that post. I would say if you leave a comment with enough words, uniquely written and most of all written to show the visitors that you know something about the subject is a pretty safe way. I have left some comments years ago on other blogs and still am getting traffic from those.
The only thing I stay away from is commenting on hubpages or Squidoo. There are far too many hubs and lenses with wrong information and it becomes useless trying to educate those who get stuck in such a web,
It's still a nice place to get back links from. It has to be relevant though.
Blog commenting is a good strategy in theory but as mentioned above, finding the top blogs that are not filled with tons of links can be one question. Today when many people find some blogs which offer commenting, they would leave their links and sometimes even without having read the articles first. Still if you manage to find some uncrowded blogs that are related to your niche, commenting on them can be useful.
There is one theory that nearly all one-way links will help websites in the long term and since blogs pages are in most cases search-engine-friendly materials and they get indexed pretty fast, so if you leave your links on the pages, search engines will pick them. It seems SEO is the only reason why webmasters do blog commenting because from traffic-building aspect, nearly no one will make click on the replies.
I do not think seo when I comment. I simply run across a lot of posts which have some wrong in it and correct it. From there I sometimes link to an article of mine which clarifies things. From there many readers share my posts elsewhere. I am not starving for traffic, but in Google I am STILL penalized for whatever reason I don't know. The site I am talking about: I have never once bought a link. It is 100% organic links and my only advertising strategy is social media.
So IMO those who posts on other blogs for seo are a little bit 1999. But let them do as they please. Askimet is doing a good job catching their nonsense.