It's usually OK. Basically, it brings up a browser window. You then find the relevant album, using AMG's normal tools. Then there's a button on the browser that grabs the info (by scaping the HTML, AFAICT) and puts it in the tags.

It doesn't work with compilations, because AMG lists them as tracks 1-40 (e.g.), rather than Discs 1 and 2, each with 20 tracks.

It works OK, but I'm not a big fan of MP3 Tag & Rename anyway.