This post is really old, but seems to be a topic which gets rehashed all of the time. The question has a major fault. The better question might be: Should I comment on blogs for SEO or for eyeballs? It makes more sense to find blogs with high readership in your market and quit worrying about follow/no-follow. Go where your market is and provide high quality content in your own sites, your guest posts, your comments, and any other materials you produce. Worry about eyeballs more and Google less.
No follow commenting is not a waste of time for the following reasons: 1. no follow backlinks will make your website look more natural in Google's eyes as a site that has only do follow backlinks may attract their attention 2. no follow backlinks may not help ranking in Google, but what about Bing? They should aid ranking there 3. a no follow link can still drive traffic to your website I hope the above points answer your question.