How accurate is Manheim's MMR as it relates to a vehicle's ACV?

[XDCX]

I was at a Manheim auction in Las Vegas a week or two ago and there was a vehicle that was on the block and the bidding had stalled at $11,000 or so.

The auctioneer was pushing to raise the bid and commented something to the effect of "Hey guys, this vehicle's $1,500 back of MMR and you all know if you're using MMR to value your cars you're...."

Before he could finish his thought someone raised their hand to bid on the vehicle and they dropped the hammer and sold the car.

To the extent the auctioneer was working for Manheim I'm guessing he was going to say that MMR undervalues vehicles making the vehicle he had on the block look like an even better deal. That said, maybe he was going to say the exact opposite.

Based on your experience, how accurate is Manheim's MMR as it relates to a vehicle's Actual Cash Value (ACV)?