I just set it up on my Pearl, and it's extremely cool. It adds a program to my apps list and after some brief setup (password, which calendars I'd like to sync, etc.), it was ready to go. I pressed the Sync Now button and off it went. A minute or so later, it was finished. My default it automatically syncs calendars whenever changes are made, so I don't have to go back to that program unless I want to change settings. Very cool.
I checked my BlackBerry calendar and sure enough, there were all my Google Calendar items. And if I make changes on the BlackBerry side, they're pushed up to Google. This is totally one of the coolest apps ever and it's about time Google gave us syncing ability with Calendar.
If you want to get into the action yourself, head over to http://mobile.google.com on your BlackBerry browser, scroll down to Sync and you're ready to go!