I was chatting to one of the sales guys at a Trek store today and he said that the IsoZone on the Trek 8.6 & 8.5 DS models had put off potential buyers.
He didn't say that there was anything technically problematic with IsoZone, just that a lot of people didn't trust it/want to risk it.
On a separate note, Trek really needs to improve the images on its website of their bikes.
All the bikes looked so much better in the store than they do in the pictures that Trek runs with.