In another discussion [webmasterworld.com] it was noted that the Bing search blog mentioned article directories as a way of generating backlinks. What are your thoughts, in general, about article directories as a link building tool?
I have found some article directories to be very helpful. What I like to do is create a logical series of information about a topic, and then each article links to the reference page on my website, and to the next article in series.
Stack the best ones at the end, and link into those in series as it makes sense for the user. The lower end directory pages support the best ones in your hub.
As others have mentioned, I use these to create my own authority hub.
Building quality inbound links to those articles then helps fuels the entire hub.
Others have added the idea of using relevant profile listings to link to articles at various steps in the process, and using social bookmarking to enhance it all.
Difficult to say 'where' it crosses the line. I would suggest as long as it provides real value, its valuable to do :)