We had addthis on our multi-million page/visitor site, and removed it after about a year. I don't know if it was draining rankings, but it was not particularly used and was also one of our worst performing widgets on the page (in terms of download time, etc.).
You can go through the code and make sure that you nofollow anything they try to stick in...
There's more than bleeding PR at stake when you add widgets and gizmos. The bleed can be plugged with nofollow but several things can't like page load times, repeating words on every page, distracting visitors from real content etc.
I doubt it killed you rankings and you will likely see the site bounce back right where it was when the "auto-evaluation-after-change" period is over, however long that lasts.
We are using AddThis in all our pr 6/5/4 pages and found no pros or cons in terms of PR. The image, js is hosted on our servers and hence doesn't affect the yslow ratings. The image links to bookmark.php which gets auto updated on their end; no maintenance on our end.
Listen carefully to what hoople says-- the actual usage of this widget is nearly nil in our experience as well. Deploy it but monitor stats, in the end I think you'd be better off with either a retweet button (if you're a business sort of site) or a facebook like button (if you're general interest), plus an email and bookmark link.
People don't really post things to delicious, digg, myspace, etc anymore, and certainly not in any large numbers, which was the original intent of the addthis/share this button.
In our case, 44% of AddThis use is to email the content. Only 10% are favorites and 9% used for Facebook. Until we can have a more robust Email to a Friend script (prior to Addthis, we used a script but was spammed to death), we'll use AddThis.
We never saw a decrease in traffic from search engines or attribute any change in PR with our use of AddThis.