Blog comment links work OK when 1) The link is a followed link 2) The page hasn't been spammed with louis vuitton/replica watches etc. links. So the task is to find a site that allows followed links with optimised anchor text but doesn't allow spam and this is becoming more and more difficult. I guess if you spam a lot of blogs some of it will stick (and thus help you rank) but as a long term link building strategy it's not really the way to go.
Most of the blogs that get spammed usually end up getting redirected/parked so it's a constant struggle to find new blogs to comment on. Also, most quality ones will have a moderation que so if you are not positing a couple relevant lines of information in your comment chances are it will be deleted out. If you were to sign up to multiple forums with signatures you don't have to worry about the post being accepted, plus you get 2-4 links per post vs one if your lucky with blog commenting.
Blog commenting is still better than forum signatures. I know it is hard to find good dofollow blogs, especially those with high PR. As a blogger, I can tell you that it is very frustrating to moderate dofollow blogs. Some people just leave a Good Work kind of useless comments. Some are even worse, they copy and paste the same comment on different blog posts. The worst are the viagra and links to Chinese porno sites.
Hmmm... duplicate thread? You don't have to ask twice if you want answer. You can also search the thread that already exist to answer your question
no, i dont think blog commenting is a waste of time. It will waste if you just find a nofollow blog and no pr to comment.
Blog comment backlinks aren't the worst for backlinking but they definitely aren't the best. Be careful not to spam too much if you're backlinking your money page because its easy to get flagged by the Big G.
I'm not sure how good they are for your links etc but I can tell you that Spam comments are so frustrating - I get approximately 50 a day on my blog and its so time consuming to go through them and make sure there are no genuine comments mixed in there.