I’m kind of late to the Fantastical bandwagon, but it really is good. You get a hotkey-summoned natural language text field for creating events, which works much better than iCal’s. And there’s also an iOS-style list of upcoming events. Rather than opening iCal, I had actually found myself pulling the iPhone out of my pocket to see this sort of list. Now I just press the hotkey. Flexibits is currently having a 50%-off sale (via John Gruber).
I also learned a good tip from the FAQ, which is applicable whether or not you’re using Fantastical:
Why does iCal automatically open when I add an event to my iCloud calendar?
If you’re using Lion, iCal automatically opens whenever new iCloud events are pushed to your Mac. To change this behavior, go to iCal > Preferences > Accounts. Select your iCloud account, then change the “Refresh Calendars” setting from Push to something else, such as Every 30 minutes. Both iCal and Fantastical will continue to sync as you’d expect.
It had been driving me crazy that I would open iCal, add an event, and quit it; and then it would auto-launch itself just a few seconds later.