Some words about our current status and what we're currently working on:

We're currently doing some tests with MindPlan on the official release of the Notes 8.5 client that was released yesterday. We did not have that Notes build version for testing before, only a code drop from the end of October and some additional patches. So I cannot say much at the moment about any issues.
The standalone versions of MindPlan on Windows and Mac should work normally, they often don't have problems with Notes client specifics, because they only need a few Java API methods for the data storage that are quite stable.
Regarding the MindPlan Composite Application plugin, here are some already known issues:

Eclipse selection implementation changed
The code in our toolbar icon to open a MindPlan from one of the MindPlan Notes views is not compatible with Notes 8.5, because there was a change in the underlying Expeditor platform. So I expect this toolbar action not to work.
This issue is already fixed (the "Eclipse selection" plugin of the Composite Application Library at www.openntf.org has the same issue, IBM is notified about it).
A workaround for this is to use the view actions "Actions/Open in MindPlan..." or open the MindPlan with "File/Open/Open MindPlan..." in the application.

NotesUIWorkspace.GetComponentViewPreference(String) is broken in 8.5 GA
This is quite annoying, because that method is a standard Lotusscript method to get static Composite Application preferences in an NSF component and in the 8.5 GA version it does not return ANY result, it only throws an error.
I think we used it in the "DocumentViewer" sample Composite Applications to hide the action bars of the Notes form. Many CAs should be broken because of this bug, so I hope IBM will release a hotfix for this issue soon (I found it two days after christmas and reported it, but it was too late to change the GA build).
I'm also seeing some other problems with the "Editable DocumentViewer" sample MindPlan Composite Application, but had no time so far to figure out what is causing this. Looks like editing a node document in the "DocumentViewer" component does not get reported to the MindPlan application. The funny thing is that is it working when I start the Notes client from the Eclipse development system.

Mac MindPlan Composite Application status
The drag and drop issues that we had during the last months has been fixed by Apple with Mac OS X 10.5.6. So drap and drop does not get interrupted any more. We had one more prio 1 issue (keyboard shortcuts not working) and talked with IBM about it. It was an Expeditor issue and not easy to fix. Don't know yet if this has been addressed, because they were afraid of the impact of the bugfix code on other parts of the product. I still need to update my Mac Eclipse development environment to the new Notes platform version.

The MindPlan standalone version on the Mac however should still be stable and the first beta to be released as official Mac version.