THIS is the update I've been waiting for. We had a forum with 25k users and a Wordpress site with 45k users with no easy way of syncing these existing accounts. The 1.0 - 1.4 version of this add-on was great for new accounts it hadn't achieved (in my mind) it's full potential until the 1.5 release. It took some time but all our users are now synced between two databases. And with the supplemental Register New Users to WordPress our site works exactly as I've wanted it too since migrating to Xenforo. Users can register in the forum (I prefer its registration over WP) and that info is sent to WP and everyone is connected.
Two minor issues did crop up: at around 28k users the syncing did hang and I ultimately had to restart it. Not a big deal, just took a little extra time as the program started from the beginning and confirmed users had already been synced. The second issue is a number of users did have to reset their passwords even after I disabled Sync User Registration as was recommended. Again, not a big deal, I just made a post for users so they knew to reset their password.
Overall this add-on is worth its weight in gold, and the developer is amazing in responding.