I have it installed. I have a Decklink Studio card.
There is no external monitoring for FCPX yet. Decklink does not currently have the extended desktop feature, as AJA does (this is how AJA hacked a ‘preview’ quality output for the Kona as a temp workaround)
Media Express has been updated in a big way. The capture tool and edit to tape interface is actually better than in FCP 7. Very nicely designed. You can easily capture media (with some metadata support) and import the media into FCP X with metadata intact. You can edit to tape Quicktime movies exported from FCPX.
There is some XML support. You can export bins from FCP 7 as XML. You can load the XML into Media Express for recapture and use in FCP X on another system that does not have FCP 7. This does not mean you can export an XML of a sequence for recapture, but its something of value.