Two issues here. The ZCO 6.0.9's features are covered in bugzilla:
Bug 53409 - Special 6.0.9 ZCO
However, although it addresses a bug I'm interested in (47647), my testing turned up
Bug 53631 - cancellation doesn't close message window, which seems to be new in this ZCO. So I've decided not to deploy it.
But, going from ZCO 6.0.5 to 6.0.8 offers a number of improvements and bug fixes. You can easily see them by looking at pm.zimbra.com for each of 6.0.6, 6.0.7, and 6.0.8, and then sorting first the requirements and then the fixed issues by Component. The biggest ones for us were 44425, 45236, 47645, 47752, and 47843.