We're are in the midst of changing over to FCP and the Sony XDCams from Avid Newscutters and DVC Pro cameras at my station. We haven't got everything from injest to playout all ironed out yet, but as far as MXF format is concerned, here is what I know about it.
FCP can import MPEG IMX inside MXF using a plugin from Flip4Mac. Presumably, you can go back the same way (or just output to tape)
FCP can real time edit MPEG IMX 50 in a sequence but not 30 or 40. You can import the media, but the editing is not real time (not sure why).
Avid's MXF option is only available for the Adrenaline line and newer. Our older 4.0 Newscutters cannot edit the IMX files natively. They have to be downconverted to Avid DV codec.
I'm learning more every day, but nothing is in the building yet.
Andy