I've been doing link building from last 2 + years & on my experience nofollow links carry very little SEO value. For many clients I've been giving high quality dofollow links and working fine in Google for boosting rank. According to Matt Cutts of Google, Google doesn't count nofollow back links, even I experimented about it by giving relevant nofollow back links to one site for few days and check very little development in ranking but once I added some PR5+ dofollow back links for that site, the rank start boosting quickly. Even I'm running some campaigns here in SEOMOZ.... I've found that sites having high value of ratio of dofollow to nofollow are ranking top. In this SEO world, I've found that many SEO experts are telling that mixing up of dofollow & nofollow back links look natural to search engines, that's true but the bitter truth is dofollow back link has much more SEO value than that of nofollow back link, especially for Google.
