Former TUAW-writer (but forever in our hearts) David Chartier tried using OS X's built-in Address Book sync to Google Contacts and had a very bad experience which ended with him turning it off almost immediately. I had a similar-but-different bad experience. In my case, OS X seemed to simply stop syncing to Google altogether, despite the fact that the setting was still enabled.
Some of the difficulty is due to the fact that Google has differences and limitations on contact information compared to OS X, which complicated syncing. I have also not been able to find any reliable way to tell when the last sync took place (it's supposed to happen every hour on 10.6), and no way to manually trigger a sync on demand.
Fortunately there's a better solution: SpanningSync. SpanningSync will sync iCal to Google Calendar and your Address Book to Google Contacts. Even better, it has a setting that I adore (shown above), called "Never Change Address Book."
This tells SpanningSync to upload changes that I make on my Mac to Google, but don't sync anything back from Google. No, this isn't "true sync" but it suits my purposes fine. If I want to add contact information, I always do it on my Mac or iPhone, not on Google. The only reason I want my contact information on Google is to use with Google Voice. If you prefer to make all of your changes on Google, you can make it a "one-way sync" that way, or make it a two-way sync.
Read on for more suggestions...