I think comment links are more valuable for building relationships with other sites and for branding with targeted audiences. I think the links themselves are diluted as far as SEO value goes.
Google knows when links are comment links, and might even disregard them all together on certain sites. If the blog has an editorial policy (needs approval for comments, or deletes spammy ones) they might offer more value. Plus the links are generally at the bottom of the page, and may send less value. It really comes down to a case by case basis. I would use the same criteria for blog posts as I do for links in general:
-Page rank or moz rank?
-Regularly cached pages from the blog in the index?
-Good content? Is it related to your site?
I would say if it's good related blogs it's not a waste of time, but don't expect to climb up the rankings just because of those links. I also wouldn't differentiate too much between no follow and do follow links. Google still tracks the no follows and having some will look organic and would still give you some of the networking and branding benefits mentioned above.
Vinnie