If you will have a chance to get juice from that particular blog, then go for it. I only consider relevant blogs as an additional specially if they are dofollow.
This is pure spam. What is the point of "Nice post" style comment? Any moderator will move it to spam folder. If you do manual blog commenting do it only on relevant niche blogs
All backlinks are benificial. Blogobot - I didn´t see anybody mentioning "Nice Post" types of comment. Make a worthwhile comment and you should get a backlink.
Irrelevant blog commenting sometime helps you gain traffic but thats too with high bounce rate. Better to do on relevant topics.
Relevancy will continue to become more important however I have posted on blogs which are not relevant and it has helped the ranking of the site. The most important thing is having a good range of links from different types of (relevant, if possible) sites.
I think that it is better to do relevant comments. Spending time doing blog comments, you will be able to point out your weak areas, and gradually your comments will start making more sense.
Yes you can still get backlinks from unrelated blogs but you might prefer links from related ones as it will benefit you more. Also don't forget that quality is still over quantity, make sure that you get backlinks from high pr pages.Besides of SEO blog commenting can also contribute to the flow of traffic to your site. Rain
Relevant or irrelevant to your blog you can get something from. That's what I'm doing when I do blog commenting. Mostly, I do commenting different from my theme. It depends on the administrator if they will approve your comment or not.
My suggestion to do only relevant backlink...google only like relevant backlink.....Always try to create relevant backlink for better seo advantages....
No. It is okay but GOogle do not want that. Comment in those relevant blog posts. In that way you will gain reputation and unique visitors.