I like the calendar that comes with the Pre. For a few days I was a little irritated that only the time was shown on the home screen (unless the screen was locked, then you saw both the date and the time), but then I realized that the calendar icon in the quick launch bar contains the day. (Insert Homer Simpson "DOH!" here.)
I use Google as my primary calendar, and when I first set up the Pre I was worried that I would have to manually turn off the public calendars that I follow every time I open the calendar. Public calendars from the SF Giants, for example, are convenient and informative in the full website version of Google Calendars when viewed on my desktop, but it can crowd out the important deadlines when viewed on the smaller screen of the Pre. In the Calendar menu, under Accounts, you can turn off the less urgent calendars in the "All Calendars" view. They are still available in the main screen to be toggled on manually as needed.
Google Maps is integrated; if you select an event, go to the menu, tap "Map Location", it launches Google Maps and you can get driving directions. This depends on the Event Location field being complete and correct, of course, and you have to have the GPS feature turned on, or you could manually input your start location.
If you have a block of free time in your day the calendar will compress with a note "X HOURS, XX MINS FREE". You can tap this note to expand the free time and tap a time to begin entering a new event at that time, or tap the note again to compress.
Now for my complaints:there's no option to change the default view to week or month, the screen does not rotate to landscape, and the only options for Reminders are 5, 10, 15, 30 minutes, 1 hour, 1 day before, or No Reminder. I can go to the full website to set a reminder for 3 days or a week, but I'd like to have the option on the phone as well.
The complaints list is minor, but it's enough for me to at least consider buying a third party application if a good one came along.