For the moment I use GPE calendar, it's ok, but I'm still waiting for something more polished (I used a treo680 before and loved the calendar of it). The widget is'nt customisable enough, often disapear, but the main problem I have is to enter the time of an appointment. It's not working with the keyboard and I have to use the stylus on small arrows, a real pain.
But for teh moment it's usable and I like the views.