Easily the most impressive AJAX application I have seen thusfar (Slashdotted). The Zimbra Collaboration Suite is unbelievably cool and I want it. Hula is promising the same and getting there, but these guys are here now and everything looks spick and span. This is really really polished.
Competition is a good thing and with all these different Open Source solutions biting at Exchange's ankles, let's see how long the old man can stand.
It provides a lot of very cool GMail-ish interaction but taken further, solid integration of calendaring, e-mail and other group work features and Web 2.0 style combination of different services such as Skype and Google Maps.
Watch the Flash demos to get a feel what is possible and then try it yourself at the site.
Another thing they have gotten down right is blogging straight forward about what is happening, what they are doing and why. These are smart people.
Update: Of course Jon Udell has more information about this. Reading the whitepapers only makes me more positive about the technological merit of the project.