I think comments are very useful in SERP as well as for PR . If comment is do follow then its good for PR otherwise its gonna help in SERP no matter if it is dofollow or no follow.
Blog comments are not going to create wonders regarding the web rankings of the sites and even if you do post regularly on a lot of the articles online, and even if the pages are closely relevant to yours, the blog comments can be like only parts of the vast link building projects and this is very important.
Trying to use more than one blog site from a different IP address for the links is a good idea.