Force Feedback is working nicely, though Forza's, ehh, stearing system is different then GT's...
It's different, at least for me, going from GT5 with wheel to Forza 3 with wheel if you know what I mean. Been a long time now since I last played GT5 with wheel so I can't really remember.
But you'll get used to it.
As for the reviews not favorable for it, I think their wrong. But of course it might not work for everyone or maybe someone didn't set up their XCM correctly. Lot's of guides on youtube.
Hoped that helped, might have sounded confusing but the feedback is working.
However, I will not promise anyone that it will work with Forza 4. Only time will tell but I guess the guys developing the XCM will update it if needed.
Now, im going to bed, had a few to much I think. 2AM and all..