Have you tried it? Do people add your content to digg and so on with it or it is still unfamiliar to readers?
It might work on certain niches, I run dozen of blogs and only few got some results with addthis. To help you with the niche, I saw good results on my SEO blog. Visitors bookmarked my blog but other niches like: computers, internet, etc had very few bookmarks on addthis.
I'm not too sure how well it works! For some people this could work. First you would have to find many friends on that particular site and start adding them to your friends list! It's seriously like getting into a cult at digg! People exchange things such as, commenting, interaction, voting, marketing and most importantly submissions of one another's content! Here's the deal so many social networking websites are popping up it's nearly impossible to keep track! The main reason the External Vote Button was created was to allow other, visitors/readers, to submit your content. These days it's being used more as a "Portal" for the author to submit their own article(s) for submission, Which make sense! I think it's good to have if people are in fact voting for your content! It shows new visitors the possibly the quality of your written content is good! There's many advantages to have a vote button as there is dis advantages! Try one out and let us know what you think and if anyone submits your articles!
AddThis is big on this bookmarking. Lot of people using it. I am sure there are some success stories. I would suggest give it a try for a few days. Then check the backlinks after few months to see how the link propagated.