My favorite calendar is the one on the Treo 650 (DateBook5 IIRC). It actually had a date selection interface that looked quite similar to the kinetic scrolling lists on the N900....but what made it better was the better agenda view, year view, ability to set events with a date but no time, more granular alarm timing, and integration with a better to-do (PIM tasks) application. Maemo 5's calendar is decent but definitely not as good and it has a few bugs.