nofollow may be useless for search engine authority, but it still provides visitor reputation and traffic if provided at the right place.
Good for Yahoo... Not PR or anything for google. They don't hurt though. I will leave a couple of no-follow comments from time to time just because.
NoFollow instructs the search engine spider to disregard that link. That means that NoFollow links will not pass any PageRank or Page Reputation through that link. This means that NoFollow links are basically worthless in terms of improving the ranking of another website. However, if you want to try to attract attention/traffic to your website by writing good comments, links from NoFollow blogs will still work fine for the human readers. If you write good comments, the readers from that blog may be inclined to see your link and click over to your site to find out more, so it might still be a source of traffic for you. It all depends on what you are trying to accomplish.
Google tells: "Link is a link, ..." It means no-follow one will drive or, better to say, can drive traffic to your website. I believe, that it's also can give your site more TRUST. But it won't pass any juice to your site and won't help increasing your rankings in Google.
If the link is in the right place regardless of the follow attribute it is worth it for the visibility. One technique I like to employ in a nofollow situation is to use my domain URL as the link and anchor, so a reader who maybe inclined can just visit the site directly.