SD XDCAM is I-Frame MPEG-IMX at 30, 40 or 50Mbit/s (I wouldn’t use anything other than 50, although 40 is pretty good).
I’ve never used it in FCP, but the idea is that you can simply transfer the files from the disk (via USB/Firewire/Ethernet) and import to your NLE as native media. No need for a capture card. Alternatively it can operate in a ‘tape style’ where you can control the deck with a 6-pin RS-422 remote and capture via SDI, but this removes the non-linear benefits of the format.
I believe to use XDCAM effectively in FCP you need this application:
Final Cut Pro/XDCAM Transfer Software
I really liked cutting with XDCAM in Avid, I hope it’s as pleasant in FCP too.