I've seen Lotus Notes, and while the windows interface is reasonable, the web interface blows goats. Plus, you can't really decouple it successfully from the absolute dogshit mailserver.
I really like the look of Web Office but the information we'll be passing back and forth is a little to proprietary and private to trust the hosting to a 3rd party.
We're an open source shop, so anything produced by microsoft is going to be treated with disdain.
What about phpGroupWare? Anybody tried that?