I've been syncing my bb contacts with Yahoo address book via the Desktop Manager until Go was available. This was a little problematic because there are a few fields that were always ignored and not synced. I decided to try the Go address book sync. I had ~ 550 contacts, with a couple new ones on the Yahoo server and a couple updated ones on the bb. Go sync duplicated 50 contacts on Yahoo and the same 50 on the bb. The newly created duplicates were missing small pieces of data, like Custom fields and extra phone numbers. I deleted one of the data-missing duplicates from Yahoo and synced again with Go on the bb. Go said it had made one change -- a contact deletion on Yahoo (which is strange as I had deleted a contact there so Go should have deleted a contact on the bb). I went to Yahoo, no additional contact had been deleted. I went to the bb address book, and saw that Go had deleted the original, good contact and left the data-missing duplicate (even though I deleted the duplicate and not the original on Yahoo). What a pain.
So far the only way I've found to reliably sync all contact data fields between Yahoo and the bb is to export from Yahoo to Outlook, then sync Outlook to the bb.
Other than that, I really like Go.