We've been using NC/UTD in a 50 person ad agency for years now, working around it's various bugs, confusing quirks and UI oversights. Many of them are still present in 5.0, including:
-Randomly needing 2-5 minutes to save the file.
-A Meeting Availability window with System 6 visuals, not OS X.
-New Appointment titles reverting to "New Appt" if you change the title, then click on a Day view window- related to synchronizing with the server.
-Various "The update failed because the server is busy" type messages that the user just doesn't need to be bothered with.
-When a user receives a meeting invite, an important factor in the Attend/Decline decision is "what do I have going on that day?" When you're looking at a meeting invitation, you can't access your calendar to answer that question.
-No legends for Sets. Which calendar in this set is blue text? That's up to the user to memorize.

NUTD5 has some useful additions- the customizeable toolbar is an OS X standard and welcome addition, and the Schedule View helps. However, it's painfully buggy:
-Click and drag from the People & Places, and you may or may not get who you clicked on. (click and drag on the y-axis, and you'll see)
-No live scroll or scroll wheel support in the name list.
-You can't start a new event of any type from this view. Why not? This is the place to answer "I need X, Y and Z in a meeting, when can I have them?", but I can't start the meeting from here.
-Schedule view functions should be available from New Meeting: View Invitee's Availability, but they're not.
-The view resets between launches, and doesn't have any ability to make presets. It would be useful to make a preset of my department from the list of the entire company, but that's not there. I have to rebuild the list on every launch.

NUTD has good intentions, and on Windows, a list of failures like above would meet expectations. On MacOS, it's a dissapointment.