ezinearticles is probably the best in my opinion. I don't submit to very many other places because i'm mostly concerned about getting traffic and I think ezine is one of the few places that has legitimate readers.
My confusion is how often? For seo, I actually like the social bookmarks, I can make comments on Blogs with my signature etc. But article writing is a chore, I plan to have 10 or 20 sites, do I have to write a different article 2 or 3 times a week for each? Thats a lot of writing! Thx Paul