I just popped a CD MC hadn't seen (not in CD Database) into the drive and MC found the metadata such as Year, Genre, Artist, Album Artist, Track Names and numbers, etc. All of that is coming from YADB, because very little is actually on the CD. I wasn't sure about that, but checked by ejecting the CD, deleting the CD from the MC CD Database, then disconnecting my PC from the network, so it couldn't look anything up. MC popped up the "CD Information" data entry dialogue asking for data about the disc to be entered. When I reconnect, click "Update from online database" all the information reappears.
Note though that when I first popped the CD in MC didn't find any Cover Art, but running "Get from internet" found the art no problem. That is because MC gets the Cover Art after completing the rip, not when the CD is first inserted. When I deleted the Cover Art and the CD from the CD Database and started again, actually ripping the CD, Cover Art was found as expected.
So my conclusion: The YADB is working fine at present.