My personal experience is that LNotes is the best groupware / workflow app out there right now. It works very well for collaborative efforts, scheduling meetings, joint "to do" lists, calendars, sharing databases etc etc.
Outlook is simply an email client, and a horrible one at that. If you just want an email client, Eudora is far superior to outlook, and isn't a virus magnet like outlook.
Also, the notes sametime client (built into lotus notes) provides instant messaging without having to "phone home". Most major corporations are not going to allow desktop apps to connect directly to some server out on the internet, so Yahoo/AOL/MSN etc messengers don't work. The sametime client works inside the firewall to allow the employees to communicate without allowing connections to those outside the company.