Dragging up an old thread:

I just tried the iPhone ActiveSync support with Google. For my contacts, where I had previously kept a careful list of them in Apple's Address Book and allowed Gmail's contacts to run wild, the result was a mess. For contacts that existed only in my Address Book, they showed up without too much mess at Google. For contacts which exist in both, no phone numbers actually migrated to Google. Had I continued with the active contact sync business, I could well have lost all my phone numbers -- clearly unacceptable.

Luckily, I backed everything up, so restoring my Address Book contents did the right thing. Not having that actively synced with Google isn't really the end of the world for me.

Calendars are another matter. Google is the primary source of my calendar. I configured the "Exchange" synchronization for calendars only, and that seems to work quite well. The only issue, sadly, is that the calendar *colors* don't come through properly from Google Calendar. See, for example, this thread. There seems to be no good way to solve this problem, save jailbreaking your phone and editing a color in a SQLite database.