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PostCalendar is definately the standard by which I judge all other calendars (though it itself isn't 'perfect' - just darn close)
I am currently using Agenda-X but am currently in the process of moving over to PiCal. The back-end is SO MUCH better, especially with respect to adding events which span dates (which has been mentioned) as well as adding events that repeat indefinately, such as holidays, birthdays, etc.;
What I am really impressed with is how the minical upcoming event block displays events which do span days. Instead of repeating the event for however many times the event occurs, in only lists it once (something even PostCalendar never addressed)
Granted it isn't much to look at at first; and if you are going for looks Agenda-X does look great. I figured though if it accomplishes more of what I like it's worth tweaking, which is where I am right now...
blessings,
Chris