Yes, I use the original MXO with my late 2009 Imac when editing in FCP 7. I have a mini display to DVI adapter on the imac so the MXO can send my image to a 23″ apple display and a 40 ” plasma (via component). I shoot mostly 720 and it looks great on both displays even with the component signal running to the plasma.
The MXO does mirror the second monitor in FCP X, but to fill the frame on the second monitor FCP expands the image to 150% and so is totally soft.
Hope this helps…
Vince