Well, they basically let you use an ATA100 drive to their full potential. In my opinion, it's not that great of an improvement over the ATA66 (though there is some), as the STR of hard drives is about 1/3 of ATA100 only.
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You have to take about 15% off the MHz for the HD overhead. So an ATA/100 will run about 85MHz in reality which is about the max speed of current HD. This is better than the 56MHz you get out of a ATA/66.
Since the difference in price between ATA/100 and ATA/66 is less than $10, I see no reason to get a ATA/66 drive and controller.