Used correctly, I believe commenting on blogs (and leaving a Website link) is a good strategy. Signature links — or links to content supporting or expanding on a point — help the reader learn more about you or find more information and are good strategy. Links plastered on a forum or blog along with a nonsense or irrelevant comment is quite definitely unwanted spam.
My measuring stick (both when deciding to approve comments on my blogs and when posting) is whether a link (1) will add value to a READER (Search engines don’t come into play in my decision-making. In my view, focus on the readers and search engine rankings will take care of themselves) and (2) is relevant to the topic. In addition, the associated comment itself must be value-added. If all three hold true, then I post a link (or approve the comment). If the link is off-topic or comment not value-added, then it’s a no-go for me.