The cynical take on nofollow
People have been calling the nofollow tag a failure for a while, most recently in Blog Spam, and a 'nofollow' Post-Mortem, which hit the geek news today. Comment spam has not exactly diminished since nofollow's adoption, which is not a good result for something introduced with the title 'Preventing comment spam'. Calling it a failure is pretty easy.
The cynical take on nofollow is a bit different: nofollow is actually a clever way to improve search engine results. While it's not going to stop comment spam unless almost everyone adopts it (including neglected bulletin boards and so on), just getting the really hot people to use nofollow helps the search engines a lot, because it's links from the really hot people that matter the most.
Persuading a few hot bloggers is a lot easier than persuading the world, especially because those hot bloggers are also hot comment spam targets and are probably quite interested in anything that can help them out. As a good symbiotic relationship, nofollow helps the hot bloggers by making it less useful to spam them and thus hopefully reducing the amount of comment spam they get.
From the cynical view, nofollow has almost certainly been a roaring success.
Despite all of this, I still like nofollow. Not for the anti-comment-spam properties (if there are any in practice); indeed, I don't bother using nofollow on links in comments here. But when search engines actually respect nofollow, it's a very useful way for steering them away from pages I don't want them requesting or indexing.
(Disclaimer: the author is not necessarily a cynic.)