I own a few of the Millennium Series -
The Grass Roots
Smokey Robinson and the Miracles
Steppenwolf
The Temptations
And have listened to several others. My experience is that they are hit or miss, some are halfway decent, but most all suffer from over compression and have been no-noised to the extreme. Some are downright painful to listen to. Unless your looking for a particular song that you can't find any other way - I would pass these by.
-Dave