Testing 10.0.3 with a Decklink card. Half-baked at best. I can see why Apple calls this a beta and only for monitoring. Dropped frames. Constantly flashes to black as FCP X is toggling states between skimming and playing the timeline. JKL timeline playback (especially at slow speeds) is useless if you are trying to view the external monitor. On the plus side, skimming works well through the external output. In general, color output is close to what I see on the viewer, though a slight gamma difference between the viewer and the output. That's normal, but it tells me ColorSync is really no better than with FCP 7.
There's also no added pulldown in the Decklink control panel. With FCP 7 the 23.98->59.94i conversion occurs in the FCP software. So, with FCP X, you have to have monitors that support 24pfs if you are working on 23.98 projects.