I thought there was a subtle interaction between .NET and VTK that caused threading problems during user interaction. The result was that widgets did not always work. Picking would fail. The problem turned out to be that I had defined an enum for EventIds that was a copy of the VTK enum. VTK, itself, changed these values from version 5.0 to the development version, 5.x, and I was using the 5.x enum for the 5.0 code.
The fix for what I thought was a subtle bug is just to use integers instead of the EventId enum. For instance, replace EventIds.EndPickEvent with the number 9. You can look up these values in VTK\Common\vtkCommand.h.
There are no other real bugs in VTK .NET, that I know of. I could do a few things to make debugging easier and wrap HWND values, but that's about it.
That means I can release a version of VTK .NET for VTK 5.0.1. I've already integrated the new release into the patch code, tested the code for the new version, and will package it as soon as I can.