I installed the Mac OS X 10.5.3 update a few hours ago, so far with no problems. One feature Apple didn’t mention in the release notes is that Address Book can now sync with Google contacts. The sync happens automatically with a .Mac sync, as well as when syncing with an iPhone. It worked very nicely for me, although I had a few duplicate entries to clean up.
I try to remember to backup before flowing a sync scheme. I've had EVERY contact get duplicated moremthan once