I use Disgby for Twitter, Google Talk, MSN, my GMail account and 2 Hotmail accounts. I used to have the Facebook Chat and a seperate Facebook news feed, running too. It works very nicely with a pretty low footprint (it used to be +-100 megs of memory, but now it's running at 43mb (plus an extra 7mb for the spell checking module).
The only issues I have with it is that it occasionally crashes while the screen saver is running (about twice a month) and that the application has no cancel button when it is connecting (or automatically downloading an update - the biggest has been 7 megs). It lately has not been able to connect to both hotmail accounts, but I think that has to do with Microsoft requiring a cookie. Maybe they'll patch it.
On the plus side, it has spell check and tabbed chats, easy to tweet in Twitter and the news feeds for Facebook/Myspace are a nice feature if you want to keep up to date and don't want to log into Facebook 20 times a day. They are multi-monitor aware for popup notifications. You can drag the window to dock, so it's nice to keep on your extended monitor docked to the side so that you can maximize your email and have Digsby present and visible.