Three limitations make this less useful than it could be...
You've got to click a button to send your tweet. It would be more convenient if Return could send it. The widget window isn't resizable (Hmm, is this a limitation built into dashboard widgets?), so the display window that shows e.g. your friends' tweets can't grow. I wish I could, say, extend the widget's height to be the height of my display. Lastly, and maybe this is also due to the Mac dashboard, there's no system notifications (e.g. Growl). So I won't know that a friend said something interesting unless I happy to flip to my dashboard and happen to see it.
It's a nice looking widget, and I really like how you can select the service, e.g. identi.ca, instead of being hardwired to use Twitter. I'm looking for micro-blogging client for my team to use at work, and this comes close to what I want us to use.