If you previously maintained your GMail contacts in sync with some application like A2G or whatever, enabling 10.5.3 Google contacts sync (in Address Book) may end in endless conflict resolution requests and duplicated contacts.

So it is a good idea to delete all your Gmail contacts before syncing. The problem is that GMail does not authorize more than 20 deletes at a time, so this can take all night if you have thousands of contacts. The solution is to use the old Gmail version, which is still available -- then you can select all your contacts and hit Delete and be done with it.

[robg adds: There's some confusion over this new feature in 10.5.3, as it doesn't show up (in Address Book's preferences) for all users. It seems it's only available for those with an iPod or iPhone, and that the contacts are only synched when you sync your device. There's a suggested fix on Lifehacker to allow Google contact synching even if you don't have an iPhone or iPod. However, according to the comments, it doesn't work reliably for everyone.]