Blackhatdamo,
No matter which sharing utility you use on your blog or website, using one in general will help increase social signals for SEO purposes. That being said there are utilities that are more feature-rich than others. The Addthis code has a lot of really great features that enhance a visitor's ability to interact with your website content. In particular are the features that allow you to promote trending topics and allowing you to connect visitors with your own social profiles. The analytics feature is also very powerful.
Some drawbacks are that I have seen others comment on is that the button itself is small and not as intriguing as other sharing buttons. The button also populates a ton of different social media networks that your readers may or may not be active on or have even heard about. Before you install addthis, you may want to give some thought to what networks your readers are likely to frequent and focus on a widget that only allows sharing to those networks. Most of the time a shot-gun approach is not necessary.
Here is a good review from TechCrunch on Addthis,
http://techcrunch.com/2007/03/05/addthis-widget-gathering-lots-of-interesting-data/
Here is a thread on SEOMoz in which commenters offer their opinion of the widget,
http://www.seomoz.org/q/widgets-like-facebook-like-and-addthis-any-negative-seo
Hope this helps you,
Shawn